concussion grenades do, yes but not for very long I don't think. Smoke grenades directly on the turret also disable them I think.
Sabotaging the commander should ensue in 'Pirates, Viking and Knights' sounds popping up for the commander, so he cannot understand teammates over microphone and Ventrilo anymore.
sabotaging armoury / barracks should stop people being able to change class at that place, otherwise you can just switch to engineer and un-sabotage it.
That would suck arse, if your team would have no engineers for some reason or another and you'd have only one barracks(like some early game). Too poweful I'd say.
or in a barracks if you change class you'll only have half health with the new class. In the armory you would have no ammo?
the "no change class" think isn't amazingly bad: firstly, you can just suicide spawn. also, wherever does an enemy scout run ALL THE WAY to the enemy base, when the team has NO engineers, and the team only has ONE barracks or armoury.