Sounds like a trick out of the AOL or NETZERO playbook.
After several attempts at getting them to stop, here's a trick I used to stop it: I changed "method of payment" to a paypal debit card account with NO FUNDS in the account. You could use a prepaid card with a dollar left on it to do the same. HULU will check to see that it's a valid card, but when they try to debit your monthly fee, there will be insufficient funds.