Sometimes it is required and you want to hide coupon form if a coupon is already applied on cart. You can use woocommerce_coupons_enabled filter to do it easily.
You just need to check if any coupon already applied by using the global $woocommerce object. If there is any coupon already applied on the cart you can disable the coupon_enabled filter by setting it false.
You can use the php code in your functions.php