Good one. Way to show who's boss there. I assume you're both in the same grade or something?
But really, the Transformers are the least important thing here (as many have said). If he's doing things like cutting power cables go to the cops. Don't be an annoying, passive agressive internet dweller who won't do anything (oh, except your brillaint 'I'll hide the childs toys!' strategy). Keep going to the cops, and make the mother vary aware of that. At the very least it might start a chain reaction of awkwardness for a few weeks, but it's better than nothing if change comes.
Tell her straight up how it is, and whats going to change.