Canceling Mobile Plan from Canada

https://forums.att.com/t5/AT-T-PREPAID/Canceling-a-sim-payment/td-p/5638749

The answer is above by @sandblaster, the 1 800 number works in Canada.

Prepaid does not have live chat. Even without the SIM card, you may be able to login to your prepaid account and turn off autopay. Use the phone number as the id, the initial pin should be the last 4 digits.  If that does not work, your options are to call or get your bank/credit card to block the charges. If you choose to call, the following info is from customer support (Even though it says inside US, pretty sure you can call the 800 number):

Customers in these situations need to call one of two numbers. 

     Outside of the U.S. - 1-314-925-6925 - The agent can call them back at their number so they don't rack up big toll charges.

     Inside the U.S. -  1-800-901-9878

English and Spanish spoken.  It's possible - depending on the agent they reach - they may be transferred within the company.I 

Thank you guys. I will try and update later

I called to 1-800-901-9878 from Canada and it worked! Thank you so much!