Honor single-use online discount codes in your retail store
Single-use online discount codes aren't designed to be used in Shopify POS. If you do want to honor single-use discount codes in your retail store, use this workaround to prevent the code from being re-used:
When a customer presents the code at your retail checkout, ask for their name.
Visit your Shopify admin, and remove the customer's name from the group (by untagging them).
You can then honor the discount by applying it in the Shopify POS cart.
You might prefer not to honor online discount codes in your retail stores. In this case you should make the restriction clear when you send customers the code.