Andrew, do you have to manually delete the spam? Like from the sql database?
What I've done before was find in the PHP the check that is done where it looks for the current user and adds the modify and delete buttons for them, and hard-code MY login name so I got those buttons for every posting. This was on a PHPBB, but I believe SMF is not terribly different.
Moderators do not have to manually delete spam. The hole in the system is that there is no way for the forum to notify the moderator group when a message is modified after a few days. The spammers do this:
Post a message that is on-topic for a thread
Wait 1,2,4 weeks
Modify the message to add their spam links
The only way for me to find a spam message that was made that way is the manually look through the SQL. I have created some queries to help me find them, but it still takes manual checking.