xD you can give the armor type a black icon (Black32x32.blp ?), give the unit invulnerability and change the Invulnerability text to space or something black. Bu there will still be the tooltip (even if you make it blank).
You might not want to give it Invul, because it affects gameplay (then you will have the 0 or whatever the value displayed), but remember, you can damage even invulnerable units via universal damage type through triggers.