I've done this for the CycleChat classifieds forums by creating a new usergroup "Classifieds".
People are promoted to this group after 15 posts and the group setting has a 10 minute window for editing (to stop people changing/retracting their offerings).
I've then made
all of the classifieds forums "Private" nodes and set the permissions in the
parent category so that it cascades down to the forums. I've revoked all privs except the "View" ones and set the edit window to zero minutes. Then in the
Classifieds usergroup I've set the privs to allow people to post/reply, upload attachments, etc. and given them an edit window of 10 mins. etc.
In the sibling forums I've set "allow" for the
View forum setting only and "Inherit" for
all other privs - which draws down the privs from the
parent and saves me having to individual set privs for each group for each classified forum.
It sounds more involved than it actually was - the key is to get the parent right.
Cheers,
Shaun