There is no option to block the gift card on the front end from F&O. The block function is used during the gift card redemption to lock the card to a specific register to avoid redemption on any other redemption. Once the redemption is completed, the POS will automatically release the lock. Sometimes if the POS loses the connection to HQ while processing the redemption, the card will stay locked. The unlock button can be used to release this kind of lock. That being, a developer can extend D365 F&O to trigger to simulate the lock.
If your requirement is to restrict further usage of the gift card, you can use the Void button which can be done after gift card purchase transaction is posted.
