Aww I hope you don't go!

Yes, you can cancel at any time. If you pay monthly then you can cancel and not be charged for the next month. You won't be prorated for the current month, but you can still use WA as a premium member for the rest of the current month.

If you go with the yearly option then the same thing applies. You can cancel anytime before your payment is due, but you won't be prorated for any time remaining, but you will remain a premium member until your membership for the year ran out.

To cancel you click on your image, click on "Account Settings" and on that page you will find the option to manage your payment where you can cancel your account.