A level restriction wouldn't help. If they needed to be level x (whatever number x is) in order to spam their advertising, then the bot script would be updated to first level them to x and then start their ads. There'd still be just as many of them, and just as much spam coming from them. They'd just be a different level while they did it.

All it would accomplish in the meantime is making it harder for genuine new players to meet up with their friends.