Any user connecting for over 300 hours a month gets flagged for tracking. If we see a pattern of three months of this, we send them a warning letter. After three warnings, if they're still connecting at that rate, they're gone. It is good business sense to drop hogs. Word of mouth is the enemy.
People have little cliques and they chat about what ISPs do.
When my average users realize that I don't allow camping, they tell all their friends. Normal' users like it because it keeps costs down. If they fall within the 'peak of the bell curve,' fine. But if they are on the high side, they pay or you can live without them. We look at our costs and see that we make a profit.