Unfortunately, as always acceptances are wholly the OP's decision. People can accept answers which are 100% wrong, or not accept answers at all, and there's nothing anyone else can do about it.
"Vandalism" on SE is normally used to refer to actions on posts. A user's posts are licensed to SE, so if they vandalise those we can take action to stop them. But the system literally doesn't allow for a way to stop people assigning their acceptances any way they want. See also Gaming system with accepts on main meta. We can suspend people for disruptive or vandalistic behaviour, but even then we can't force them to re-accept answers they've unaccepted. And of course once the user is deleted we literally can't do anything to them - though you may be interested in this feature request.
It's regrettable, but I don't think there's really anything we can do to resolve this situation.