If you find yourself in this situation, the answer is to create a 0 (null) payment: Select both the voiding and the voided invoice in the Cash->Payment (or Cash->Receipt) screen. This will clear the invoices against each other with a net-zero result.
I've voided an invoice in my system. Now, instead of *no* open invoices, I see *two* open invoices listed: the voided and the voiding one. How to remove both invoices from the list of open items?