Once you have logged into your account follow these steps:
1) Click on your avatar / profile picture in the top right corner of the screen.
2) Choose 'Settings'.
3) Click on 'Billing Info' at the top right corner.
4) Click on the red 'Cancel' button.
5) Click 'Cancel this subscription'.
That is it! Once you've completed these steps you should receive no further charges for the course. Your access to the course material will remain until the end of the current billing cycle.