To turn off Store Credit in Magento, disable the Magento 2 Store Credit Extension. To do so, go to General Configuration Settings -> Select the Disable Extension option from the settings menu.

Read this How to Configure Store Credit Extension in Magento 2 Store to setup this extension.

Here are the few more resource to help you get most out of this Magento 2 Store Credit Extension.

• Maximizing revenue with Magento 2 Store Credit Extension
• Raise Loyalty & Returning Visitors with Magento 2 Store Credit Extension
• Store Credit in eCommerce: A Key to Customer Retention