Block members from starting new threads not working

[Macsee]

I would like a certain usergroup to be blocked from starting threads in a particular forum.

I have gone to the Admin CP > Forum & Moderators > Forum Permissions. For this forum and this user group I've set "Can Post Threads" to No.

But users from that usergroup are still able to create threads in this forum.

Is there some other setting somewhere that needs to be corrected?

--------------- Added [DATE]1342627659[/DATE] at [TIME]1342627659[/TIME] ---------------

I've discovered this does work for some members in the usergroup, but not others.

This usergroup is based on postcount. Members get promoted to this group on acquiring 5 posts. Members who move up to this usergroup from Registered Users seem to be blocked from creating new threads in this forum (as intended). Members who've been manually "demoted" to this user group seem to be immune and they can still create threads.

Why do the usergroup restrictions for a forum work only on some members and not others? This seem to be a vB bug. Can anyone confirm?