Perhaps advertising signatures could only appear in:

• Economics
• Marketplace
• Trading discussion
• Meta
• Politics & Society
• Beginners & Help
• Off-topic
• Archival
• Altcoin Discussion
• Announcements (Altcoins)
• Mining (Altcoins)
• Marketplace (Altcoins)

This would leave:

• Bitcoin Discussion
• Development & Technical Discussion
• Mining
• Technical Support
• Project Development

Advert-spam free?

Although I guess this list is highly subjective, and we are back at the question of "does this really incentivise spammers to post less"?