You can do set ttl to 1, but any "wise" user with some networking knowledge will pass trough that, by any of several posibilities of routers that can change ttl themselves.
You can use connection limits, but even that won't help too much.
Changing ttl will stop only casulal "smarties" from sharing your internet connection.
It's all about what are you selling: bandwidth or number of stations.
In the long-term, you'l see that selling rock stable capacity will get you clients.
You can put an aditional network address on your router, and give them all that network for use, put a queue on the whole network, to limit to whatever you want, and you can see then how many computers they use, if they don't use any router. If they do, you have your money for what you sell, anyway.