The etiquette part is how to handle frustration with what appears to be a design flaw or two without being rude to people who think that I went to school with them when I was living in another state. The good side is that I am sure that they were not the ones bullying me back then.
I think that handling your own frustration isn't really an etiquette thing at all--that's sort of internal, no?
The only part that *is* etiquette is how to reply to the people who click on your name as a former classmate. And of course you can choose a path that *minimizes* your internal frustration and still be polite.
My vote: Just "ignore" them or whatever it is the site offers. Or block them. Whatever's available to you. Don't even bother replying.
For one thing, what if it's spam?
But even if it isn't--when you ignore/block them, you'll fade out of view. You don't owe them any more than that, really--social media isn't the same thing as real life.
It's just a mistake, and it's easier to handle than a telemarketer call.