Simply visit the Subscribe tab and change your selection to "Do Not Subscribe" and the tab should disappear.
Regarding why the tab appears even if subscriptions are disabled:
To maintain compliancy with the CAN-SPAM Act of 2003, users who are already subscribed must be provided with a method to UNsubscribe.
If your subscription system is turned off, it does not accept new subscriptions. However, users who are already subscribed to a page will see the Subscribe tab for that page, for the only purpose of unsubscribing. If you visit the tab, you will see the only option is Do Not Subscribe (and the current selection to avoid confusion).
Similarly, if the user has any active subscriptions at all, they must still have access to the "My Subscriptions" page, for the sole purpose of being able to unsubscribe.
Even though your subscription system is currently turned off, if the user ever received a subscription notification in the past, they might not be aware that subscriptions are turned off now, and they have no way of knowing if you will ever re-enable subscriptions in the future (even if you don't plan to). If they cannot find an option to manage their existing subscription, they can technically report your site to the FCC for noncompliance.