Forum Deny permission to post in forum by usergroup
 

We have a complex setup of usergroups on our site. This is a setup that I believe has been working for awhile, but now seems to be broken.

Basically, we have a forum. In that forum, users of Group A CAN post to that forum. Users of Group B CANNOT post to that forum. If a user is a member of both A AND B, he SHOULD NOT be able to post in that forum.

What we are experiencing, however, is that a member belonging to both of those groups can post in the forum.

I can write a plugin to fix this behavior, if necessary, but I and the rest of the team feel that it was working as we need it to work in the past. In the Forum Permissions section, Group B has been explicitly denied access to the forum, and Group A still has its default permissions (Which is to allow the posting)

Is VB operating as intended and I need to write a plugin to accomplish this?

Thank you in advance.