actually if Square-Enix's account services are anything like SOE's, if you stop the credit card from being billed (i.e. calling your credit card company) they'll suspend your account (after all...credit cards expire and get stolen, it wouldn't be fair to cancel all the accounts just because the credit card gets cut off) so if you ever want to play again, you just resubmit your credit card information
i dunno, i did that with Everquest 1, so it may work