The ISP that I used to work for had something called a BCU - Bandwidth Control Unit. It was an expensive piece of equipment, about $5000 US, that would sit and look at the MAC addresses of the equipment that were trying to get through.
We were doing a hi speed wireless internet service in a rural area that didn't have cable or DSL access for anyone. Phone lines were so bad, some people couldn't even get dial up.
We offered three different speeds, depending on how much the customer wanted to pay, and controlled their speed from the BCU. We also used to BCU to prevent anyone from getting onto our service without paying.
I'm sure that there are other, less expensive, options available.