Prevent redeeming a gift card via the URL
When a gift card email is received, there is a Redeem button which contains a URL with the gift card. When a customer clicks on the button, it will automatically apply the gift card to the cart.
If you do not want to allow a gift card to be applied to the cart via URL, perform the following steps:
1. Install the free Code Snippets plugin: https://wordpress.org/plugins/code-snippets/
2. Create a new Snippet with the following code:
add_filter( 'pwgc_allow_applying_by_url', '__return_false' );
Note: if you prefer, add the code to your functions.php rather than using Code Snippets.