There's no way of setting up a minimum post limit so it'll mean manually deleting the posts and as I seem to spend most of my time deleting and editing then I don't want to make too much work for the other mods.
I'll leave it up the the forum members. If they want to deal with someone with only one post then thats up to them.
In a spanish forums, they make the trade forum visible only for certain User Group. If you want to sign up on that group, you need to have 30+ posts.
So, a new user sign in, and must ask for signing in on that group in order to enter the trade forum. If a moderator/administrator checks the number of posts of that new user and is under 30 posts, then the petition will be discarted, else he will be accepted in the 'Trade Group', and will be able to see the Trade Forum.