It is possible to remove the Microsoft subscription for your account at the following link:
If you have any other questions or issues in the future please don't hesitate to get in touch.
Most of the mmo I've played deal with subscriptions in the same way, and the easiest option is to cancel your membership as soon as possible after purchasing it (i.e.: buy 1/3/6 month > cancel it one minute later before forgetting about it). You can still play your time; all you have to do is log back in on their website if your account is locked.