one registration for one PC

Hi,
I have a registration page ang once a customer get registered, its free as a trial for 7 days. But after 7 days they have to pay to use some features in the application. this is checking using an email as a user name. But if the customer using different emails, they can use these featuers again as trial. I want to stop registering with different email from the same PC. how can I do that. Please give me some ideas and examples. is MAC address or some thing else we can use for identifying the same customer??