I've only seen a couple BST posts in there. I expected much more.
Instead of limiting our users and not letting people reply, I'd rather just remove the BST posts. Help us out by reporting the ones you see.
I don't know who we wouldn't allow to post in there anyway. I've seen new people make some great posts and more experienced users goof up by posting an offer. Plus, it looks like people starting threads are as guilty as the people replying and it would defeat the point if we limited who was allowed to create a thread.