Prevent Access to Custom 404 Controller

[eluser]fjamal[/eluser]
[quote author="Aken" date="1344492058"]What's the difference? It's still going to come up as a 404 page, just like any other 404 page. The user sees the exact same thing.[/quote]

Just curious on how to prevent user from accessing a specific controller. Anyhow, its not a big deal; it could be done via sessions.