Wire cutters are too blunt. That results in one of two things. Either a new machine gets brought in or they get a new power cable (depending on the design of the machine).
Instead, she wants to get ahold of a wall adapter:
Do some digging and see if you can't find a prank wall adapter that grounds the power line. (Otherwise you have to do some tinkering to make one). Then it looks like normal, and nobody could guess that the intercom isn't actually plugged in. Whenever Oonagh wants, she could just remove the prank plug and everything works the way it's supposed to. Oonagh can pick a few days each week for the intercom not to work.
If Oonagh plays her cards right, she can stretch that out for years without anyone figuring out the machine's broke, and buying a new intercom. They'll just think they're idiots who can't make it work right. Even when they do get a new machine, she'll still have the prank plug.