This is the one I'm talking about ...http://custom.simplemachines.org/mods/index.php?mod=585
Now it says it's not designed for large forums, but what's a large forum? Some of the forum I go to will have a 1000 or more online at once. This one seems to average 150-200, big but not massive.
This mod seems to have a lot of controls, but I haven't studied it in depth. Refresh controls, turn off guest posting etc.
It's just an idea anyway, not a must have.