I know it's a joke, but vandalism doesn't help. It makes you look like the jerk.

I can only second/thrid/fourth what NocoleC and others   have said. A dozen friends can move a LOTTA stuff in one day, and a Uhaul, medium size, holds a LOTTA stacked boxes. It's worth buying the professional ones as they will then be all the same size and shape, stacking more neatly. You are collecting boxes, yes?

I see no reason you couldn't haul everything out of the garage and say you are freeing up the space.

By all means have more than one witness whenever you have to interact with these people. For moral support, for documenting purposes.