Create a script that uses a flat file containing known usernames that are spammers (like this
one), and have the script run once an hour to delete anything posted under that username in any forum. Any of the site admins could add to the flat file instead of deleting the account.
That way the spammer thinks he/she is successful, but their spam quickly vanishes. Deleting the account just results in the creation of a new one that takes time to identify.