For a resigned member, what do I do to zero out the remaining balance?

If there are no payments against the invoice yet, void the invoice.

If there are, create a credit note on Xero against that invoice. See this article for reference: 

• Creating a Credit Note against an Invoice

No payments on invoice: void the invoice. 

Payments on invoice: create a credit note in Xero against the invoice. 

To create credit note: 

1. View the Invoice in Xero. 

2. In the Invoice Options, select "Add Credit Note" 

3.  The form will automatically be filled out. Check all the values and update the amount if necessary. 

4. Click Approve.

5. The Credit Note will be synced to DotGolf within 5-10 minutes.