Restrict subscription ONLY to logged in users

I want to prevent 'strangers' (ie users who are NOT logged in) from being able to subscribe to a Membership Subscription.

I've placed the [renewform] and [subscribeform] shortcodes to only display when a user is logged in, however I've noticed that someone can go to mydomain.com/wp-admin/admin-ajax.php?action=buynow&subscription=1 to bypass and subscribe.

Is it possible to prevent them from doing so?