So implement inability to post if you don't have 25 posts. Lots of forums have that. That is the best solution. If managing the classifieds forum is too much for Grendel to manage then it might be time to appoint someone to manage only the classifieds forum. I'm sure lots of people here would be happy to volunteer. It takes like 10 minutes a day. It's not like this forum gets massive traffic that there's a new thread in the forum every minute.
A 1 month ban because they glossed over the rule isn't helpful to anyone. If you look at his posts he had potential to be a helpful member. By acting so irrationally you're effectively just pushing away new members. A short ban like will make it so people don't forget about the site, and they will still remember the repercussion of breaking the rule. 1 month? They'll forget the site even exists. It's a lose-lose scenario.