Dear Sarmed,
As I see ur problem is related to this controversy http://forum.mikrotik.com/t/hacking-hotspot/9775/1 and u can disable default forward from ur wireless card to isolate ur client.about the person he told u all this information about ur users and there MAC its very simple cause u can do that with a simple scan program and then change ur MAC address and use the service as we discuss in the link above
Regards,