When creating a door that doesn't open in your map, it's a good idea to ensure that the player knows it's locked - usually by playing a "locked" sound when the door is touched. This is very simple to do but makes your map more interesting simply by being more engaging and interactive.
Even though your door will never open, you will still need to make it a func_door entity. See the links above for details on how to create doors. Remember that entities do not seal your map, so even if there's nothing behind your door you will need to add another brush to avoid causing a leak. The secret for a fake door is to set some properties:
Name: Set the name of the door to something that you won't ever trigger. A name like locked works well.
Locked Sound: Set the locked sound of the door to something other than "None".
Other properties should stay as the defaults.
That's all there is to it! As long as you never trigger the door from another entity, giving the door a name will prevent it from being openable in any other way.