You can log on to your account on swtor.com, then manually go to this link;
https://account.swtor.com/user/subscription/Cancel
You can cancel from there even while banned.
Many legitimate users had to use this method because of a bug (ahem...?) on the subscription page on the final free day that prevented people from cancelling subs (when they had 0 free days left but payment hadn't yet been taken, the cancel subscription link vanished - but you could still cancel by manually visiting this link).