It's there a way to still void Credit Memos when you have this preference enabled: Setup > Accounting > Accounting Preferences > Void Transactions Using Reversing Journals in order to allow voiding of a stale AP check in a different period than original AP check date. We want the ability to void checks AND Credit Memos.

this might give an idea: https://nserpsolutions.com/order-to-cash/customer-payments/voiding-customer-payment-using-reversing-journal-entry/