How to “cap” downloads

Hi, I live with 3 other people and we all share the same internet through a wireless router. Only problem is that we can only download a maximum of 95GB a month and somehow within the first week or 2 it is reached. I was wondering if anyone knew a way to ‘cap’ each computer at 20GB for example so that once a person has downloaded that much then they cannot download anymore.

My router is a D-Link WBR 1310 if that helps at all.