An ISP in my area actually has a rather clever system. When you get to 80% of your bandwidth cap, they send you an email notifying you that you're at 80%. If you go over your bandwidth cap for the month, the next time you open a web browser, you will be re-directed to a page where you can make a decision. You can either pay $5 for every extra 20 Gigabytes you use, or you can choose to not pay anything but for the rest of the billing period your internet speed will be drastically reduced. I like this approach, where you can pay for the extra speed, or you can just stick with the free stuff. More ISPs should give you that sort of flexibility.