As I mentioned a few comments up, I don't think you can implement a countermeasure like that without violating the TOS. And even if you were technically in the right, you'd be right back in the situation of automated bots thinking you were up to something fishy and banning you.
Which part of the TOS do you think this would violate?
AFAIK you're not under any obligations to show Adsense ads to all users, or all of the time. I know quite a few people who only show Adsense ads to search engine visitors, for example, or block ads for people arriving with certain referrers.
