Is it possible to make an entity unbreakable on contact with an area?

[Cruzore]

by unbreakble, do you mean unbreakable for enemies? If I understand you right, you want to go through a door, which will lock itself so the enemies can't break through. If you mean that, you will have to deactivate the grunt and refrain from using sounds, particles etc.
I haven't seen a function in the engine scritps page to set a door breakable/unbreakable, but I guess you can work around that by setting the prop health to 100 once it reaches less.