Anyone figure out how to block Anonymous posters to Y!A?

No and it's total bulshit that someone that's blocked can simply hit anon and post on your question then if they have you blocked you can't even respond to what they post on your question. In a lot of ways Yahoo set the site up for Trolls, this is one of them.

You can't since there isn't a feature to do that.

It is not possible to block anonymous users.

You can't,there is no option to do so.