To delete payments, go to the Billing tab > Payments. From here, select the payments you wish to delete by clicking on the dots beside them, click the top drop down menu, and select Delete Payments. 

You can delete an individual payment by clicking on the drop down menu across from the payment and selecting Delete Payment. 

You are also able to delete a payment in the Billing tab > Overview, by going to their account > Click the down arrow in the payer's billing account > Click on the garbage can across from the payment. 

Note: you cannot delete any payments made through Sandbox.