Possible simple anti-spam measure?

I've noticed that many (though not all) of the bots that are registering at my site for some reason change their time zone to -12 (this is a forum running at GMT). I'm assuming that this box has a different meaning on other forum systems.

There are generally speaking very few people living in this timezone. Now while I don't agree with banning it altogether, would it be possible to have either a plugin or mod or a built-in function that simply requires some extra validation for users who sign up and change their time to -12? It could potentially filter out a massive amount of junk.