I update game time via prepaid time cards. I still have time left on it could that be why its not working?
As Owlet mentioned, there is no subscription to cancel if you use prepaid cards and simply not adding more time will cause it to expire naturally when the time runs out, putting it into a Frozen state.
To add to that, there isn't any way to Cancel and get reimbursed for the remaining time or save the remaining time for later when you wish to renew.
You can "Cancel" and send your message to Blizzard why using https://www.worldofwarcraft.com/account/cancel-account.html. That won't stop the game time, but lets you provide a brief "why" as to your reason for cancelling. (I remember the box lets you type more, but needs to be either 250 or 500 characters or less for the Cancel to be successful.)
One last thing, criminal account thieves do sometimes gain access to inactive / frozen accounts. If your account is compromised and you come back more than a few (three or so) months later, you might not be able to have a full restoration done. For this reason, even if your current intent is to never come back to WOW, it is highly advisable to make sure the account is secure.
That can be as simple as making sure the computer you're on is clean then doing a password reset and setting it to something you use nowhere else. Or you can go further and create a special email address and set up Parental Controls with no available play time. Checking your Battle.Net email / account management once in a while to be sure no activity has occurred can be good too.