How to disable a specific JavaScript alert

You can use the Greasemonkey add-on to rewrite the alert function:

// ==UserScript==
// @name        Catch JS Alert
// @namespace   http://igalvez.net
// @include     http://*
// @version     1
// @grant       none
// @run-at      document-start
// ==/UserScript==

window.alert = function(message) {
    if(message == 'Adblock detected, please consider disabling it') {
        console.log(message);
    } else {
        confirm(message);
    }
}

The way this works is as follows:

If the alert box's message matches "Adblock detected, please consider disabling it", then discard it to the JS console (it will not be displayed). Otherwise, display the alert box as a confirm box.