You could always make it more difficult on them, and install the Visual Confirmation mod. It's contained in the /contrib/ folder of the phpBB 2.0.10 package. Also, if you're having an issue with the "proxy enabled" breed of trolls you can install something like Block Open Proxy Registraints
to weed them out entirely
As for the second question, are you wanting to delete users or the entires in the member list?