Properly monitoring debit and credit memos issued can lower the risk of theft thru A/R fraud. Use ID107 to see credit memos created and who created them. Use ID112 also, the difference is ID107 shows all voids and credit memos. ID112 only shows misc credit memos and vendor invoice voids.
It's also important that you require employees to say why they voided a transaction, ensure you check that box in Tools / Options 'Require Notes When Voiding Transactions'