Wifi control access for cafeteria

[Before reading this you have to know that I´m Spanish and my English is not perfect] Hello, if you come to a cafeteria with Wifi you can be here for a long time using the Wireless conexion at the cafeteria. I want a software who use a password typed by the waiter for each client to give them an access to the net for a defined time (30 minutes for example), I prefer give a full access (every port), but I will accept too a 80 port application. I will install the software on a computer, not on a router, this computer will be connected to the net and will have a Lan board. The program will be used under Windows. I want this program to work only if a key is activated (this key can depend of the MAC address). Thanks! I think I have explained the project well, if you have any question you can send me a message.