// Hide ALL shipping options when free shipping is available
add_filter( 'woocommerce_available_shipping_methods', 'hide_all_shipping_when_free_is_available' , 10, 1 );
 
/**
* Hide ALL Shipping option when free shipping is available
*
* @param array $available_methods
*/
function hide_all_shipping_when_free_is_available( $available_methods ) {
 
  	if( isset( $available_methods['free_shipping'] ) ) :
		
		// Get Free Shipping array into a new array
		$freeshipping = array();
		$freeshipping = $available_methods['free_shipping'];

		// Empty the $available_methods array
		unset( $available_methods );

		// Add Free Shipping back into $avaialble_methods
		$available_methods = array();
		$available_methods[] = $freeshipping;

	endif;
 
	return $available_methods;
}