Paid users shouldn't be able to cancel their signup.

Users shouldn't be able to cancel their signup once marked as paid. The following is an example of why not:

- If a user pays for their signup via paypal, their details are logged into the database and their signup status is changed to "paid" or equivalent.
- If that user then cancels their signup, their payment details are still logged into the database and the admin doesn't know without manual intervention

Furthermore:
- If a user then signs up again, then they are rightfully marked as having paid (having being compared to the signup_pay table)... but their signup status remains as "registered". This is because the signup status update is done during paypal processing, after Instant Payment Notification. Again, the admin has no clue if they have paid.