After I took Mike's suggestion into consideration I came up with this: A little cabinet (pedestal if you like) in the neighbourhood fitted with a keypad and a display. On this the customer types in the pre-payment code from the voucher. If it's a metered service, there are meters and a simple computer inside which keeps track of consumption. If it's a flat rate service, there are instead simple relays which turns the power off when the customer has used up the days he has payed for, e.g. 30 days.
From this little cabinet the cables run out to the houses. This way I don't need a pre-payment terminal at each house which should save money and make it harder to bypass. Sure, someone can still break into the cabinet, but it's not very hard for the utility to find the culprit. Just follow the wires...
