Voiding Refunds is the same as voiding a transaction and it would only be possible if the batch hasn't settled yet with your payment gateway. If your batch has settled you will need to process the charge again to reclaim the funds.
If your batch has not yet settled with your payment gateway, you can use the Void button within the Transaction Record for the Refund. You can locate it by the Transaction Type and Status.
Click the Void button on the top and this will Void the Refund prior to settlement.