There have been a few posts re: banning already, but here's a suggestion:

• Create a “standard” user group called Bozos (included in standard SPF) with zero privs
• Add a new capability to Moderators, i.e. moving a user to the Bozos group
• Ensure that the name of the prior user group membership is maintained, allowing the Admin to override and/or another Moderator to roll back if/when necessary
• Prohibit Moderators from overriding the Admin

And aside from that, just pointing to the available Moderator tool, i.e. looking up the IP from which a given post was submitted, which then could be plugged into Lester Chan's WP-Ban (assuming that's set up) as well as the recommendation to consider using also the very fine Bad Behavior plugin which – when set to “strict checking” – really minimizes opportunity windows for vandals and bots, as BB won't allow proxy connections at all. Not such a good idea to set BB to “strict” when you have a high-traffic forum with tons of *cough* idiots *cough* who can't resist installing browser toolbars with funny fluffy smiley icons and those handy advertising popups included, but for the sane part of the internet, it's pure gold.