If you are on the older 2 year Spotify promotion, the Premium subscription will cancel itself automatically at the end of the 2 year period. It's not possible to cancel it before that.
If you are on the 6 month promo you can cancel it via Fidohub.ca. All you need to do is enter your number, and you'll receive a code. Enter that code back on Fidohub and from there you can cancel the subscription.