Ok, you have a subscription to someone and you have automatic payments every so often? I think that I may have the solution.
Go to Paypal My Account > History > Basic Search, click on the Transaction details for the payment that you want to cancel, and see if it has a Subscription Payment Sent (Unique Transaction ID #xxxxxxxxxxxxxxxxx)
In reference to:xxxxxxxxxxxxxxxxxx. If it does, you should be able to click on this and be linked to a page where you can cancel it.