Banning Mac Address On Thomson TG782T?

The problem i'm having is that i'm using a 100gb capped internet plan, but after 1-2 weeks it gets all used up. After checking to see who's accessing the internet there's 2 unknown mac addresses who are constantly accessing the internet. Whenever this happens, the internet becomes considerably slower (i'm guessing they're torrenting or downloading off our internet).

I've even tried changing the wpa password but they're somehow still accessing the internet. So i've been looking for a way to ban mac adresses but i can't seem to find the option on the router.

Here's a screenshot:

YES, i know there's an option that says "Allowed on WLAN", but i cannot un-check the box for some stupid reason.

I really don't know what to do! Is there some kind of command i can type in the command prompt or something to ban mac address manually?