Conditional for prefix help

[qpurser]

Hope somebody can help me with the following:

I have a forum called classifieds (forum id10)
This forum has 2 prefixes ("only members" and "open to all")
Now I only want to show in this forum (thread list in this forum) the ones with the prefix "only members" to the user group called "members" (user group id 9)

I guess I can do this with a conditional but not sure how to.
Also where would I put the conditional?

Thanks
Michael