the current ISP model

So change the current ISP model. Analyze your traffic, and design three new subscription plans:
- Standard. Is slightly cheaper than you current standard plan, includes enough monthly bandwidth that 60% of your customers can use it with out hitting the limit. Grandma's connection.
- Pro. Slightly more expensive than Standard, higher limit, works for the next 30% or whatever of your clients. Basically everyone between grandma and hogs.
- Elite. Pays pr. gigabyte, minimum of X gb, with X set so high that it's always more expensive than Pro. Tells the hogs to f*** off or pay up. Offer a lower cost pr. GB at night or whenever you have excess capacity.

Silently accept a certain (high) temporary excess usage for the the standard and pro plans, so the grandkid leaving a bittorrent open on grannys computer doesn't get a huge bill. If traffic is consistently too high, send an e-mail or give them a call, and tell them to slow down, upgrade, or be billed for excess usage.