Remove all shipping options except Free Shipping if Free Shipping is available.

gistfile1.php

// Hide ALL shipping options when free shipping is available
add_filter( 'woocommerce_package_rates', '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'] ) ) :
		
		foreach ( $available_methods as $available_method ) {

			$available_method = get_object_vars( $available_method );

			if ( $available_method['id'] != 'free_shipping' ) :

				unset( $available_methods[$available_method['id']] );

			endif;
		}

		endif;
 
	return $available_methods;
}