If you're on the free thirty day trial there is no way to "cancel it," other then simply waiting for the trial to expire. If you've paid for it, again, the same thing applies. You can however cancel your subscription so you are no longer charged for premium; this article should help. If you no longer wish to have an account, you can contact Spotify Support to have your account deleted.
If you are on a trial period your account will automatically downgrade to a free account once your premium trial expires, unless as I recall, you've entered credit card information. If you haven't provided payment information then you have nothing to worry about. I'm assuming that since you are unable to cancel your premium subscription that you have either done it before unkowingly, or that you don't have any payment information in the system, so again, you should be safe.
If you have provided payment information and you are unable to cancel the subscripton, then I would contact support. Hope this helps.
If there is no information to charge, then your account will simply lapse to a free account.
Can't charge what they don't have.
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