Once logged into the WM GUI:
- From the Configuration options, select "Wireless"
- From the "Wireless" options in the left pane, click on the "Association ACL" option
- In the right pane create an ACL
- Add a deny with the specific MAC that needs to be blocks and specify a precedence
- Add a permit to allow all others to access the WLAN and specify a precedence
- Save and commit changes