Why not just a "trashbin" thread? Crap posts get moved there, make it read only, a script cleans it up every week/fortnight/month?
Users get x strikes and you're out (where x 1 > average of moderators' temperance), i.e. more actively enforce the "we give warnings" part. An invalid/continually bouncing e-mail address gets a PM, if a PM is not answered in y days, then a public reply, if no response after z days, account suspended (suspended in case user on holiday etc.) After final quarantine period account deleted.
