You've got it the wrong way round, you've just given every role administrator rights for that forum.
You need to create an AccessMask called No-Access (if there isn't one already, depending what version your using). Which basically won't have anything ticked. That give every role the No-Access Permission apart from the group you want to access it.
"It's a case of RTFM.. the only problem being we don't have a manual!"
When I post
FP:Mek in a topic, I'm leaving my footprint there so I can track it once I get into coding/supporting. (Yes I stole this off Ederon
)