gistfile1.js

// ==UserScript==
// @name      DontStealMyShurtcut
// @namespace  http://ryo1kato.github.com/
// @version    0.1
// @description  enter something useful
// @match      https://docs.google.com/spreadsheet/*
// @require    http://code.jquery.com/jquery-latest.min.js
// ==/UserScript==

function disableCtrlKeyCombination(e)
{
    //list all CTRL + key combinations you want to disable
        var forbiddenKeys = new Array('a', 'e', 'n', 'p', 'f', 'b', 'd');
        var key;
        var isCtrl;
        if(window.event)
        {
                key = window.event.keyCode;     //IE
                if(window.event.ctrlKey)
                        isCtrl = true;
                else
                        isCtrl = false;
        }
        else
        {
                key = e.which;     //firefox
                if(e.ctrlKey)
                        isCtrl = true;
                else
                        isCtrl = false;
        }
        //if ctrl is pressed check if other key is in forbidenKeys array
        if(isCtrl)
        {
                for(i=0; i<forbiddenkeys .length; i++)
                {
                        //case-insensitive comparation
                        if(forbiddenKeys[i].toLowerCase() == String.fromCharCode(key).toLowerCase())
                        {
                                e.stopPropagation();
                                return true;
                        }
                }
        }
        return true;
}
$("div.cell-input").on('keydown', function(e){ disableCtrlKeyCombination(e) } )