I just happened to be on there...
As per paypal:
"You can cancel only unclaimed payments. If your payment is complete, please contact the recipient of the payment to request a refund.
Unclaimed payments can be canceled for the following reasons:
* A payment sent to an email address is not associated with a PayPal account. Once the email address is added to a PayPal account, the payment is automatically credited to the balance of that account.
* A payment is sent to a recipient with an unconfirmed email address. For security reasons, PayPal can only credit payments sent to a confirmed email address.
If you cancel an Instant Transfer, PayPal cannot cancel the withdrawal from your bank account. The amount will be credited to your PayPal account balance.
1. Click theMy Account tab.
2. Click the History subtab.
3. Click Cancel in the Action column of the transaction in question.
4. Click Cancel Payment.
5. You have successfully canceled the payment. "