You Always use Credit memo.

"How do you write them off if you're on cash basis?"

The invoice is going to show, washed away by the credit memo, which Also Shows. You use CM to have all this controlled by date, appear to affect Sales reporting, and if you need it to, you also get to control how this affects Sales Tax reporting. Set the date on the Credit memo = your date of Write off.

You don't use Receive Payment and pretend Banking, you don't use JE for Names, so never for AR.

Please see my attachment for how Cash Basis works Fine for this.