Actually I think that allowing it on older nodes is very
relevant. It appears that vote-bots are being used on older nodes, and people should be able to use this system to help identify them. But the challenge should only be to *votes* made in the last week for exactly the reason stated.
Also note that you would only respond on the basis of votes made and challenged. I suspect that a very small portion of nodes would ever get challenged, so any individual voter only would be challenged on a small number of votes. Also note that unexplained ++ votes would be lost as well, so a node in the positive would lose rep for a challenge. That should discourage people challenging nodes casually.
As for abusive people, well their nodes tend to get reaped, and once reaped you no longer own it and so would be unable to challenge.
BTW a good point made in private to me is that it is important that when people respond to the challenge to have the recipient told whether that was a ++ or a --. Otherwise half vote ++, half vote --, but everyone says, "I liked it and voted ++."