This article assumes you want to fully write off a Membership payment due and cancel the Membership.
1. On the financial side, assuming you are running cash-based accounting then you will simply not record the membership payment receipts into your accounts, ie this is cash you won't receive.
(If you are recording accruals then you will have already accrued for the membership income in your accounts and will have a Debtor Account balance. In this case, you will need to create a <negative> Receipt transaction against your “Membership Income” (or equiv) Cost Code and the Debtor Account. This will reduce your income and the debt balance.)
2. In the Membership system, you will also need the delete the billing line(s) in the Members' Payment Plans - see below - This will remove the liability balance against the Member. If you also need to cancel the Membership, go to the Membership Details tab: enter the Cancellation Date and click then Cancel Membership button.
Please also see this alternative method: https://clubtreasurer.com/blog/community/postid/165/