It seems like the actual problem here would be solved, like many of the forums problems, by eliminating signatures. No signatures, greatly reduced incentive to spam, no need to have bots deleting suspected spam.
Or, even better,
- ban any member participating in a signature campaign (and delete all his post history)
- ban any member which participated in the past in a signature campaign (and delete all his post history)
- ban any member which ever wrote "thank you" or similar words (and delete all those posts, but to be sure you teach him a good lesson, better delete his entire post history as well)
- enforce other gulag rules each possible time, in order to make sure that nobody ever dares to break the rules.
Seems a bit much