In large deployments (as stated in the piwik FAQs) piwik administrators separate UI and trackers (different servers). The only way I have found to disable the UI/API on the trackers is to put them in maintenance mode. The message displayed when the tracker is in maintenance mode may not be 100% accurate because the administrator might not actually be performing maintenance, they might just have the UI disabled.
I would like to see the ability to specify a “custom maintenance page/URL” in the config file which would be displayed in the event that the tracking server was in maintenance mode. This way a more appropriate message could be displayed if a user accessed the piwik tracking URL directly.