Reverse purchase invoice

How to reverse purchase invoice which amount already been paid through bank last month

You find the transaction in bank tab and delete the payment. Then you can go to purchase invoice and delete the actual invoice. I presume that the transaction got cancelled. What I prefer to do is create a purchase order and only after I have bought the product and received it and am happy with the product do I actually then convert PO to Purchase Invoice.

You said the amount was paid from your bank account. Was it actually paid? If not, do as @dalacor says. That will remove payment and purchase invoice.

If you actually paid supplier and now you have a credit with supplier. You can either create a debit note under Debit Notes tab. Or you can re-categorize the payment to go to Supplier credits account, then delete the purchase invoice.

I am doing a bank reconciliation and have 1 payment to reconcile before I can finalise this month. It was originally paid through my debit card and then all or some of it has been reversed - I have yet to find out. I will have to call the bank on Monday to find out who the supplier was as there is no mention who/which company did the reversal on the statement. Once I know who it was from, how do I enter this reverse payment into my accounts?

The exact procedure will depend on exactly what happened. But generally speaking, a reverse payment is just a receipt. So Bank Transactions => New Bank Trasaction => Receive money.