If the forum is set to force everyone to subscribe, users viewing a discussion will still see an 'unsubscribe to this discussion' link. Clicking on the unsubscribe link won't actually unsubscribe the user - it will go to a page that says 'Everyone is now subscribed to this forum'.

If the forum is set to not allow subscriptions, user are able to subscribe to the discussions in that forum.

If the forum forces subscriptions, there shouldn't be an option to unsubscibe from discussions. If the forum doesn't allow subscriptions, there shouldn't be an option to subscribe to discussions.