If your transaction was sent using Payment Requests, and the customer has NOT made the payment, then yes, the transaction can be cancelled by you.
Please follow these steps:
Go to the dashboard: dashboard.pomelopay.com
Click on the specific transaction that you want to cancel (its status should be Pending if your customer has not paid)
Click on “CANCEL” at the top right corner
Confirm that you want to cancel when prompted
The payment request would then be cancelled and the link which was sent to your customer would no longer be valid. You can go back to Payment Request on the dashboard to send a correct request.