The Store Credit toggle field on the Customer Database Custom Data screen (stored as value 32 in the Flags field) is enabled by default and can be disabled to prevent the use of Store Credit Sales Payment Types for individual customers. If the Customer's database record does not have the Store Credit toggle field enabled (turned ON) then Store Credit sales payment types cannot be used for that Customer and if used, a warning sound is played, and the message "Use of Store Credit is disabled for this customer." is displayed. For Export/Import/Reports the field is labeled "StoreCredits" and is at location 86 (except for Import where its location is 83 due to the 3 Read only Loyalty fields).
See:
Edit Store Credits