And then in the smallprint:
Additional traffic over the maximum average daily use will be invoiced at $X/Mbyte.
With that daily average calculated on a window of the billing period and the maximum set at something the majority of your customers are well below, somthing reasonable like 300Mbyte/day.

Pass it through marketing and legal a few times and no customer will notice till/unless the leachers get their first bill and start to protest: so you "gently" squeeze then out first (DMCA/copyright complaints sounds like an easy way to get them out and not have to take the heat) and be happy with the bulk of the rest of the customers.

The fun part: your competitor who stays last "truly" unlimited will end up with just the leachers and will have a _far_ bigger problem on his hands.