You cannot do it from the GUI. You can do it from the commandline of AMP using the on_controllers directive combined with the "stm add-blacklist-client" directive:
on_controllers "stm add-blacklist-client <mac address>" on the commandline of AMP will login to all of your controllers and add that client to the blacklist.
You can of course see what clients are blacklisted on each controller by running;
on_controllers "show ap blacklist-clients"