Well, this is a PHP Coding Forum, not Tucows; but let's see if I can think of something...
Deleting the account is too simple, too straightforward.
Unsubscribing them from all those spamhausen that keep shoving unwanted junkmail at them would probably backfire.
Find something extremely personal in them and post it to several dozen off-topic newsgroups - could be good for a laugh.
Direct his email to /dev/null.
Pull the plug.