When a bad debt could not be collected after trying all possible collection methods, than a record adjustment must be made to reflect the uncollected payment. This is known as a Bad Debt Write Off. A general journal entry must be made to reflect the uncollected sale.
The basic for the accounting procedure is the same; the actual procedure depends on the accounting system you are using. Here is a helpful video courtesy of Doris Cahill on how to write off bad debt using MYOB accounting system.