Hi,
I don't know the actuall logging level you need, but why not enable logging at the least visible, level 1. Create a filter for a specific client, and then gradually increase the lgogging level until you see the client being prevented from joining. It may be that this process is only visible using a debug command, something like debug client <mac address>.
This is certainly valid on the WLC, but I have never tried it on a wireless router but is worth a try, just make sure your debug output goes to the buffer and not the console.
HTH
Mike
