For Online Computer Support, Ask a Computer Technician
You can cancel if you know the email account that it is connected to using these instructions
this page has the same instructions with a link to click on to log in
Or you can get a call from xbox support to cancel it by clicking the box on this page
or if nothing else works you can call the number on the back of your credit card and tell them that you dispute the charge. They will put the money back in your account and probably issue you a new card so that it cant be charged again next month.