Free Accounts – Stopping Repeat Signups

How can we assure that anyone who does a free trial does not create another free trial when there’s expires.

Here’s how we want our site to work:

1. We have 1 month, 3 month, 6 month, and 1 year memberships.

2. Everyone gets the first 15days free, then their account is charged.

We have spoken with PayPal and they tell us we can do this. Basically, everyone will prepay but not be charged until their 16th day. If they cancel prior to that, they won’t be charged.

3. If they cancel, is there a way for the system to not let a user with the same email address signup again?