I understand. I've been running forum site software for about 15 years now. One thing I do need to also consider and look at is how many people do go to post on For Free/For Sale as one of their first posts. Or more importantly, of those, who stays around to add to the community (I don't prioritize people who take and don't give anything back).
I generally prefer to not block content from being read. Even for high risk spam sources (certain subnets/countries), I allow the ability to read content, but not register/post. But it might be reasonable to do for this purpose. I'll look into that method. It might be possible to apply different rules to the "Newly Registered" group (which is how I think phpBB does it).
Anyway, I do need to look into it. It's more a matter of time than anything else. Being an Info Sec guy by trade, I'm particular about user authorization settings.