It cannot be done.
This is something which I mentioned well over a year ago - and of course I was laughed at, called numerous names and then accused of just scaring people.
The average monthly fee that an ISP collects from somebody just about covers around 8-10gb of BT core network bandwidth.
So any user using more than this amount per month is actually costing the ISP money.
At the moment there are plenty of "normal" users around who don't use anywhere near the 8-10gb of bandwidth so most ISP's can afford to have a cap above and beyond this.
The old theory of "For every bad user we've got 10 good ones" rings true.
However as people start abusing the no limit connections and downloading 24/7 ISP's are being forced to cap.
Even the big boys such as Zen are being forced to introduce caps - those that don't will either collapse as all the bad users will migrate in their direction or will soon find their business model will fail.
In fact as with dial-up there is only one ISP that currently don't cap AND have said in interview after interview they have no intention of introducing one and that is AOL.
Sure there are some around right now that don't have a cap - none of these have said that caps are not on the horizon.