This script goes through and removes all members from a Facebook group. Replace the ID in the second line with your own Facebook ID!!! Then navigate to the members tab of your group and copy/paste the entire code into the javascript console. Script tested in Chrome. Removes 10 members a minute!

facebook-mass-delete-group-members.js

var deleteAllGroupMembers = (function () {
	var excludedFbIds = ['100005315525880']; /*########## Replace number between single quotes with your own Facebook ID. DO NOT REMOVE THE SINGLE QUOTES! ##########*/
	
	var deleteAllGroupMembers = {};
	var usersToDeleteQueue = [];
	var scriptEnabled = false;
	var processing = false;
	var memberCount = document.getElementsByClassName('_5bv4')[0].lastChild;
	
	deleteAllGroupMembers.start = function() {
		scriptEnabled = true;
		deleteAll();
	};
	
	deleteAllGroupMembers.stop = function() {
		scriptEnabled = false;
	};
	
	function deleteAll() {
		if(scriptEnabled) {
			queueMembersToDelete();
			processQueue();
		}
	}
	
	function queueMembersToDelete() {
		var adminActions = document.getElementsByClassName('adminActions');
		for(var i = 0; i < adminActions.length; i++) {
			var gearWheelIconDiv = adminActions[i];
			varhyperlinksInAdminDialog = gearWheelIconDiv.getElementsByTagName('a');
			var fbMemberId = gearWheelIconDiv.parentNode.parentNode.parentNode.parentNode.parentNode.parentNode.id.replace('member_','');
            var fbMemberName = gearWheelIconDiv.parentNode.parentNode.parentNode.getElementsByClassName('fcb')[0].textContent;
			
			if(excludedFbIds.indexOf(fbMemberId) != -1) {
				continue;
			} else {
				usersToDeleteQueue.push({'memberId': fbMemberId, 'memberName': fbMemberName, 'gearWheelIconDiv': gearWheelIconDiv});
			}
		}
	}
	
	function processQueue() {
		if(!scriptEnabled) {
			return;
		}
		if(usersToDeleteQueue.length > 0) {
			removeMember();
			
			setTimeout(function(){
				processQueue();
			}, 6000); /*########## Sets total time period between deletions in milliseconds. ##########*/
		} else {
			getMore();
		}
	}
	
	function removeMember() {
		if(processing) {
			return;
		}
		
		var memberToDelete = usersToDeleteQueue.shift();
		var gearWheelHref = memberToDelete.gearWheelIconDiv.getElementsByTagName('a')[0];
		var memberCell = memberToDelete.gearWheelIconDiv.parentNode.parentNode.parentNode.parentNode.parentNode.parentNode.parentNode.parentNode;
		
		processing = true;
		
		gearWheelHref.click();
		setTimeout(function() {
			var popupRef = gearWheelHref.id;
			var popupDiv = getElementByAttributeValue("data-ownerid", popupRef);
			var popupLinks = popupDiv.getElementsByTagName('a');
			
			for(var k = 0; k < popupLinks.length; k++) {
				if(popupLinks[k].getAttribute('href').indexOf('remove.php') !== -1) {
					popupLinks[k].click();
					setTimeout(function() {
						var confirmButton = document.getElementsByClassName('layerConfirm uiOverlayButton selected')[0];
						if(confirmButton != null) {
							confirmButton.click();
							memberCell.parentNode.removeChild(memberCell);
							
							var newMemberCount = parseInt(memberCount.innerText.replace(',', '')) - 1;
							memberCount.innerText = newMemberCount.toLocaleString();
						}
						processing = false;
					}, 2000); /*########## Sets time period in milliseconds to wait to click on 'Confirm.' ##########*/
				}
			}
		}, 500); /*########## Sets time period in milliseconds to wait to hit 'Remove from Group.' ##########*/
	}
	
	function getMore() {
		processing = true;
		more = document.getElementsByClassName("pam uiBoxLightblue  uiMorePagerPrimary");
		if(typeof more != 'undefined') {
			more[0].click();
			setTimeout(function() {
				processing = false;
				deleteAll();
			}, 5000); /*########## Sets time period in milliseconds for the next batch of members to load. ##########*/
		} else {
			deleteAllGroupMembers.stop();
		}
	}
	
	function getElementByAttributeValue(attr, value) {
		var elements = document.getElementsByTagName("div");
		
		for (var j = 0; j < elements.length; j++) {
			if (elements[j].hasAttribute(attr)) {
				if (value) {
					if (elements[j].getAttribute(attr) == value) {
						return elements[j];
					}
				} else {
					return elements[j];
				}
			}
		}
		return false;
	};
	
	return deleteAllGroupMembers;
})();

deleteAllGroupMembers.start();