Help needed to prevent scamming

On my Membership page which is shown when non subscribed users try to access member only content I show my payment buttons at the bottom of the page so they can sign up through PayPal and access the content if they so want.

One of these buttons is a “trial” option for £5 for 1 weeks trial.

I also have a £10 a week subscription button as well as monthly and yearly subscription buttons.

However I have had a couple of occasions now where a member has signed up for the 5 day trial option then come back a month later and used it again and got another trial – which I obviously don’t want as in theory they could keep using the trial option instead of the £10 weekly option to get weekly access half price forever.

I cannot think of a way to stop this.

I have put strong wording in the T&C’s that say you are only allowed one trial and won’t be refunded if you cannot login and so forth but if the user is not logged in at the time I cannot wrap the buttons in the [s2] code snippets as the system doesn’t know who they are.

I have wrapped the £5 trial option in a [s2] snippet so that any existing member (whether they are still subscribing or just logged in without access to the membership only pages) cannot see the £5 trial button. However this relies on the user logging in first.

However if the user does not login they can see the button and therefore re-sign up.

Really I need some way of linking ISP/IP address/user-agent and storing it so that when a user comes to the site and they are not logged in I can still work out which user they are and then auto-log them in and NOT show the button. Or just NOT show the button because I can ID the user in someway.

I know this is not a fool proof way of identifying the user as IP addresses change (AOL, Proxies, DCHP etc) but I cannot think of a way to prevent users from taking advantage of my trial option if they are not logged in.

Can anyone help or suggest ideas on preventing this from happening please.

Thanks in advance for any help given.

Rob