[kh99]

Well, you could truncate the mailqueue table in the database, but if you do you'll also lose any legitimate emails. If you know a little sql you can probably come up with a query that will only delete the spam (if there's some keyword in the subject or body you can use).

Also, in the admincp under Email Options, you might want to increase the "Number of Emails to Send Per Batch" (although if someone sent tens of thousands of spam messages, maybe you won't need to increase it once those are gone).