You can't do this with Adblock Plus if you normally have it turned on for every site, but add an exception for stackoverflow. There's no way to add an exception to an exception.
Here is a greasemonkey script; unfortunately, the ad still flashes on screen for a second :(
// ==UserScript==
// @name Remove case-of-the-mondays
// @namespace http://example.com/
// @include http://stackoverflow.com/
// ==/UserScript==
var crap = document.evaluate(
"//img[@src='http://ads.stackoverflow.com/ads/case-of-the-mondays-careers.png']",
document, null, XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, null
);
for (var i=0;i<crap.snapshotLength;i++) {
var img = crap.snapshotItem(i);
img.parentNode.removeChild(img);
}
Seriously, does anyone like that ad? Can we just kill it please?