The setting that controls this is a business rule. You may designate whether Accounts Payable always applies, never applies, or asks you before applying open Credit Memos to new Invoices for all Vendors.

To set this business rule:

1. In Accounts Payable > Configuration, click Business Rules.
2. Select Invoice
3. From the "Automatically apply open credit to new invoices" drop-down menu, select ask each time, always, or never.