The easiest way to void a payment applied in QuoteMachine, is to do so directly in Lightspeed Retail POS (R-series)

From QuoteMachine, identify the payment you wish to remove. This could be directly on an invoice, or a credit memo, as seen below.

Now, click on the link to open this payment in Lightspeed. There, you can void it.

After about 10 minutes, the payment will be deleted from QuoteMachine.