how to cancel a subscription?

There are instructions on this page for managing and stopping auto-renewing subscriptions : http://support.apple.com/kb/HT4098

It won't let me cancel my zoosk subscription

What happens when you try to cancel it, and you've tried it via both your iOS device and your computer's iTunes ?

If you can't turn it off then you could try contacting iTunes Support and see if they can do so for you : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption