I think I need some more clarification. It sounds like you have a smartphone and may have had the tethering feature turned on? If that's the case, and your roommate no longer lives with you, it's unlikely that he's using your data package because his cell phone would have to be in range.
Or is his cell phone part of your account? If his number is under your name, all you would need to do is a Transfer of Billing Responsibility to put his number in his name. You can do this online or you can get in touch with our customer care department.
I hope at least one of these answers was helpful. If not, please respond with more details, and we'll get this sorted out.