add_filter( 'woocommerce_coupons_enabled', 'mcw_disable_woocommerce_checkout' );
function mcw_disable_woocommerce_checkout( $coupons_enabled ) {
global $woocommerce;
// Check if any coupon applied and disable coupon if any coupon applied
if ( ! empty( $woocommerce->cart->applied_coupons ) ) {
return false;
}
return $coupons_enabled;
}