In 3.x and below, we could simply use overriding permission masks to bar certain members from a forum or forums.  In 4.x the permission masks are gone and while you can set permissions with groups, the primary group permissions always govern.  I cannot use secondary groups and we DO NOT want to alter the primary group.

What other options are there other than purchasing the Ban from Forums mod??

I gave in and got the mod.   IPS should have this built into the software.