Can't disable REST API in current theme

I have tried to disable REST API using both the plugin ‘Disable REST API’ and by manually insert this code in the functions.php of the child theme:
add_filter(‘rest_enabled’, ‘_return_false’);
add_filter(‘rest_jsonp_enabled’, ‘_return_false’);

But without any luck. The developer of the theme (Ultra Web Studio) don’t know what to do, so they directed me in here.
Can anyone help?