You can unsubscribe over on the subscription page.
Click on "View Your Options" at the bottom, under "Manage Your Subscription", and you should see the option for Cancellation right here.
Strange. Could you try another browser or incognito/private mode? If that doesn't work, please use the online contact form and someone at Spotify can help get this sorted. If you get an automated response directing you back to the community or to the help pages, please reply to it to speed things along.