In the GUI...
Configuration Tab
Wireless tab
Association ACL - Create one to deny the MAC. Be sure to do a permit at the end for all others (permit 00-00-00-00-00-00 FF-FF-FF-FF-FF-FF precedence 1000)
Once created...
WLAN Tab
select the SSID to apply ACL to
Expand the list of options for the WLAN
Select Firewall
then look for association ACL and enter the name of the ACL you created above to deny the MAC. Apply and save.
How to do via GUI.....
I prefer to use the CLI as it is much quicker. Download the Command reference guide for WiNG. All CLI commands are there. Very easy to use.
Good luck.