I finally managed to make it work :
function remove_searchfield() {
$tag = 'epl_frontend_search_field_select'; // the name of the hook
$function = 'render_select'; // the function you wish to remove
$priority = 10; // must be an integer
remove_action ( $tag, array( Easy_Property_Listings::instance()->search_fields, $function ), $priority);
}
add_action( 'init', 'remove_searchfield');
Thanks Jacob for the help 😉