Add users to ignore list in channels. One separated black list for each channel.

slack-ignorer.user.js

// ==UserScript==
// @name         Slack Ignore List (19.11.16)
// @namespace    https://gist.github.com/shtrih/be922d9229925bb52617
// @version      1.4
// @description  Add blacklists to channels
// @author       shtrih
// @match        https://*.slack.com/messages/*/
// @grant        none
// ==/UserScript==
/* jshint -W097 */
'use strict';

// https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver
(function ($) {
    function getActiveChannelId() {
        var pubChanId = $('[data-channel-id]', '#star_container').attr('data-channel-id'),
            privChanId = $('[data-group-id]', '#star_container').attr('data-group-id')
        ;
        return pubChanId || privChanId;
    }

    function getMemberIdMenu() {
        return $('a.member_preview_link', '#menu_header').attr('data-member-id');
    }

    function getMemberIdMessage(elMessage) {
        return $('.message_content a.message_sender', elMessage).attr('data-member-id');
    }

    var Ignorer = (function () {
        return {
            set: function (channelId, memberId) {
                localStorage.setItem(channelId + memberId, 1);
            },
            get: function (channelId, memberId) {
                return localStorage.getItem(channelId + memberId);
            },
            remove: function (channelId, memberId) {
                localStorage.removeItem(channelId + memberId);
            }
        };
    })();

    var targetMessages = document.querySelector('#msgs_div'),
        targetUI = document.querySelector('#client-ui'),
        menuIgnoreUser = $('<li id="member-ignore" role="menuitem">\
<a><span class="menu_item_label">Ignore user</span></a>\
</li>'),
        messageHiderHandler = function (divMessage) {
            var classList = divMessage.classList;
            if (classList && !classList.contains('unprocessed') && classList.contains('message')) {
                var channelId = getActiveChannelId(),
                    memberId = getMemberIdMessage(divMessage),
                    isIgnored = Ignorer.get(channelId, memberId)
                ;
                if (isIgnored) {
                    classList.add('hidden');
                }
            }
        },
        messageReveal = function (memberId) {
            $('ts-message.message.hidden', targetMessages).each(function () {
                if (getMemberIdMessage(this) == memberId)
                    $(this).removeClass('hidden');
            });
        },
        refreshHider = function () {
            $('ts-message.message:not(.hidden)', targetMessages).each(function () {
                messageHiderHandler(this);
            });
        },
        // Скрывает новые сообщения от юзеров
        observerMessageHider = new MutationObserver(function(mutations) {
            mutations.forEach(function(mutation) {
                if (mutation && mutation.addedNodes instanceof NodeList && mutation.addedNodes.length) {
                    Array.prototype.forEach.call(mutation.addedNodes, function (node) {
                        var $node = $(node);
                        if ($node.is('.day_container')) {
                            $('.day_msgs', $node).children().each(function () {
                                messageHiderHandler(this);
                            });
                        }
                        else {
                            messageHiderHandler(node);
                        }
                    });
                }
            });
        }),
        // Добавляет пункт о блокировке во всплывающее меню юзера
        observerUserMenu = new MutationObserver(function(mutations) {
            mutations.forEach(function(mutation) {
                if (mutation && mutation.addedNodes instanceof NodeList && mutation.addedNodes.length) {
                    Array.prototype.forEach.call(mutation.addedNodes, function(node){
                        if (node.id == 'menu') {
                            var $node = $(node),
                                channelId = getActiveChannelId(),
                                memberId
                            ;

                            // если не меню юзера или меню юзера не в общем чате
                            if ($node.attr('data-qa') || !channelId || !(memberId = getMemberIdMenu()))
                                return false;

                            $('#menu_items', $node).append(menuIgnoreUser);
                            $('a', menuIgnoreUser).off().on('click', function () {
                                // console.log('ignore user', memberId);

                                $('#menu').remove();
                                Ignorer.set(channelId, memberId);
                                refreshHider();
                                showIgnoredMembers();

                                return false;
                            });
                        }
                    });
                }
            });
        }),

        // Обработка списка участников
        targetSidebar = document.querySelector('#details_tab .channel_page_member_lists'),
        observerSidebar = new MutationObserver(function(mutations) {
            mutations.forEach(function(mutation) {
                if (mutation && mutation.addedNodes instanceof NodeList && mutation.addedNodes.length) {
                    Array.prototype.forEach.call(mutation.addedNodes, function (node) {
                        if (node.nodeType !== Node.ELEMENT_NODE)
                                return false;

                        // console.log('observerSidebar', node);
                        processIgnoredMembers();
                        observerSidebar.disconnect();
                    });
                }
            });
        }),
        ignoreIcon = $('<i/>', {
            class: 'ts_icon ts_icon_flag',
            style: 'display: inline-block; font-size: 20px; line-height: 20px; position: absolute; top: -8px; left: -12px; color: red; cursor: pointer;',
            click: function () {
                var parent = $(this).parent(),
                    memberId = parent.attr('data-member-presence'),
                    channelId = getActiveChannelId()
                ;

                Ignorer.remove(channelId, memberId);
                messageReveal(memberId);
                parent.remove();
            }
        }),
        showIgnoredMembers = function () {
            $('.presence', '#channel_page_all_members').each(function () {
                var self = $(this),
                    memberId = self.attr('data-member-presence'),
                    channelId = getActiveChannelId()
                ;
                if (Ignorer.get(channelId, memberId)) {
                    self.after(
                        self.clone().attr('title', 'Ignored. Click to cancel ignore.').html(
                            ignoreIcon.clone(true)
                        )
                    );
                }
            });
        },
        processIgnoredMembers = function () {
            var observerMemberList = new MutationObserver(function(mutations) {
                mutations.forEach(function(mutation) {
                    if (mutation && mutation.addedNodes instanceof NodeList && mutation.addedNodes.length) {
                        Array.prototype.forEach.call(mutation.addedNodes, function (node) {
                            if (node.nodeType !== Node.ELEMENT_NODE)
                                return false;

                            // console.log('observerMemberList', node);
                            showIgnoredMembers();
                        });
                    }
                });
            });

            showIgnoredMembers();
            observerMemberList.observe($('[data-section-name="members"] .channel_page_member_lists').get(0), { childList: true });
        }
    ;

    observerMessageHider.observe(targetMessages, { childList: true });
    observerUserMenu.observe(targetUI, { childList: true });
    observerSidebar.observe(targetSidebar, { childList: true });
}(jQuery))

1. Ignore user:

2. You can click flag in user list to cancel ignore: