I have already wrestled in the past with throttling bad behavior from RSS scrapers. I currently also have a governor in place that I call "excessive host usage" which prevents any one IP address from making thousands of queries, running up my Web bills, and hurting my servers. It's been quite effective, as out of control non-major robots or other scripts tend to be very out of control, and my governor clamps them down right away.
Now someone could set up their own server to run the queries from my server and then pass them to their own AJAX pages. I can use a governor to prevent that. Individual users could make some number of requests over reasonable periods of time, but thousands of requests of a certain type would indicate behavior that needs to be blocked.