The only way to do this would be to make some members moderators. Which would allow them to delete not only ads, but edit/delete/close any topic or forum.
I appreciate the offer, but right now that's a step I'm not willing to take. If the spam problem gets worse, then I may consider it. For now it's not a huge deal if there are a few spam posts up for a few hours before I see them.
Deputizing moderators opens a whole new can of worms.