Making custom taxonomies into radio buttons? & preventing adding new?

I'm curious if CustomPress will allow you to create a category-like taxonomy that's a radio button? I'd like to prevent users from selecting more than one of a couple custom post types. This plugin here: has that functionality. But I'd prefer not to run both it and CustomPress at the same time (afraid they'll conflict with each other, plus it's just one more plugin). If this isn't a feature of CustomPress, I'd like to request that it be added.

Also, is there a way to lock a custom taxonomy so the person posting can't add any additional options? Or is that something you'd just do with permissions?