You can try to fix them any way you want but they will find a work around one way or another. Throttling their connection on the ISP side is the only true way to stop bandwidth hogs.
Simply set a xGB tiers and then start throttling the connection back. As they go up further into additional tiers, throttle their connection further. Then reset it to normal and let the process start over.
Then they can market higher throttling points for power users that should pay more to use the additonal bandwidth they are requesting.