[bairy]

You don't have to disable the whole of AJAX, just the in-thread editing (when you click edit, it's the text box that comes up).
You don't even have to disable that if you don't want to, it's just that anyone who happens to trigger the limiter will have the warning phrase appear, header n all within the thread, which looks really horrible.
If anyone knows a way to force a page refresh and then show the error message when you click "save" on the in-line editor, I'll stick it in.

At the moment I don't know how I could disable AJAX for those members.

As for vB plugins, you're outta luck. The places I've edited don't have hooks anywhere near them.