Make Blogs appear on the "blog directory" only after a certain number of posts and remove

Hi,
I'm running a site with multisite and buddypress. My issue is that 90% of the blogs that are created stay empty, but they all appear on the "blogs" page (www.domain.com/blogs).
I would like to find a way to keep those empty blogs out of the list so users can browse more easily through the sites.
And eventually i'm looking for a way to remove those empty sites if they stay inactive too long.
I'm trying to find in the plugins but my searches didn't return anything interesting for now.
Thanks