RIght now its either AJAX on or off for the site, or just for all courses (which essentially is for the site). Back in the days when we had AJAX settings by user we used a "Quicksets" block so the user could turn AJAX off when it wasn't working (most of the time) and turn it back on when they really needed it, like moving a section twelve places.
My experience with AJAX right now using FF and Chrome, latest releases is AWEFULL (wish I could politely put more 'LLLs' in that word). After ten attempts I might be successful moving a label that contains only a graphic, or it might take me more or I might end up re-creating it because it JUST WON'T STAY PUT. Same thing with blocks only worse. If you watched how often AJAX actually successfully fires, you'd want to turn it off most of the time too.