Yes sometime i have the same thoughts, i should just quit replying, but i am kind of addicted ;D
I understand your problem, that why my previous reply to you was mentioning to "remove any previosly granted authorization", the person which can get access with all its machines, can do that because before you gave him that privilege, so, to fix that, you now have to remove those rights from its account(s), blocking them again, and only insert the MAC address you would like to allow on that list.
If you can't find them, i would suggest to start blocking everything again, then only make change on the MAC addresses's list.
I hope that's clear enough.