You will need either your Lumia phone or PC to cancel the subscription.
On your phone, open Nokia Music, select settings and sign in to your Nokia account. Tap MixRadio+ > cancel subscription > tap Yes to confirm the cancellation.
On your PC, open MixRadio at http://mixrad.io and sign in to your Nokia account. Click your account name in the upper right top corner > select cancel subscription > tap OK to confirm the cancellation.