They won't go "Well, we won't supply you any more" and get a new customer, they'll take you to court for the money.
Well... no. They'll just suspend your account for non-payment until you pay.
And that's part of the problem... the service provider ultimately has the upper hand, since the customer needs that internet connection a lot more then the service provider needs the remaining $7. As such, the reality is that pretty much any customer who tried that kind of stunt will panic and promptly pay up as soon as they realize that their internet has been shut off.
This service provider advantage is also why utility companies (gas, water, electricity, etc.) can so easily get away with charging outrageous "reconnection" fees, just to flip a switch and turn you back if you should happen to miss your payment date for some reason. As such, it's that much more important that the service provider be held accountable for their system of measurement. A "proprietary" system of measurement just doesn't hold water.