Add this code to your System - Ad Block Reactor plugin, in a new "Change Action" item.
document.addEventListener('ABREvent',function(e){
var label = Joomla.getOptions('plg_system_adblockreactor').lastreport===null?'detected':'changed';
var value = window.adblockreactor.vars.lastreport?'enabled':'disabled';
ga('send', 'event', 'adBlocker', label, value);
},false);
You should see something like this when you're done:
After your users have a chance to react to your requests to turn off their ad blockers, you should see some data like this in Google Analytics: