How do I stop a visitor from double dipping into my free trial?

Hi,

I've got my site set up to a 60 day free trial. After 60 days of during the 60 days, a member can upgrade to my paid subscription.

When the visitor views their subscriptions, they get the option again to "upgrade" and move to the free trial. Two questions:

1. What is there to stop them from "upgrading" and signing up for the free trial again when the paid subscription expires?
2. What is there to stop them from using several gmail or other email addresses in order to remain on the free trial forever?

Is there an IP address log somewhere that stops them from doing this?

Thanks,
Louise