I've just broken unbrakeable brick wall with a circular saw. Just drag the saw in god mode towards the wall. Also seems breakable by drill.
Items that where broken by this never respawn and still broken even untill level restart (stop + start)
I've discovered it when I tried to cross a bridge, made of walls, on a truck with saws instead of wheels.
I am host. Here are steps I've rechecked 1 minute ago:
1) Launch game
2) Enter multiverse as host
3) Load/create machine with circular saws or drill
4) Add brick wall
5) Disable it's physics
6) Start level
7) Crush the wall (make the machine hit wall with drill or saw)
And after level stop this wall doesn't respawn, because game still thinks it is unbreakable.