You can only cancel a pending payment if its gone to a non existent email address and has a cancel option as the payment can't ever complete.
If its gone to an email address that does exist but not yet linked to a paypal account then as long as its not claimed by that email address (by them setting up a paypal account) then it will be time out 30 days post payment.
If its pending awaiting authorisation to a company then you ask the company to cancel your order and not to capture the pending payment...then again it will time out 30 days post payment.
If its a pending echeque then once its cleared you can ask the seller to refund it back to you as you can't cancel a pending echeque.
Lastly if it completes then you can open a dispute for non receipt of item OR not as described (for more info read buyer protection).
Advice is voluntary. Kudos / Solution appreciated.