You can set a user’s notification level for a topic through the API. You’ll need to generate an API key for the user first, and then make a POST request to /t/$topic_id/notifications with the user’s api_key, api_username, and the notification_level in the body of the request. To avoid exposing the API key, the request will have to be made from the server, not the client.
The easiest way to deal with this would be to only show the Follow button to logged in users.