A good headhunter reads your profile carefully, and contacts you if and only if they have a position that's a really good fit.
A bad headhunter finds your email address online, and cold-call emails you a random position that doesn't match your interests at all.
A *terrible* headhunter gets passive-aggressive about the fact that you ignored the previous email, sends you an "oh, didn't you get my previous email? Let me tell you about the position again" followup, and then gets defensive when you call them on it.
(Yes, I know -- first-world problems. But I don't believe I've ever gotten so angry at a recruiter before -- I wound up letting loose a full-bore flame this time. If she writes back yet again, I'm going to have to find someone at the company to complain to about harassment...)
Well, eventually we stayed friends, and she promised to send to me any really good stuff she may come up with. (Have not heard from her since.)
They are, mostly, humans.
The truth is, every job I've ever gotten has been in one way or another through networking. (Even the one that *did* come through a recruiter -- she remembered me from a previous interview, which I had gotten through a friend, and thought correctly that I'd be a good fit.) But I try to maintain good relations with a small cadre of recruiters who I've found, over the years, to be serious, professional, and respectful of my time and interests -- those are the folks I will actively contact if I'm looking, and who I will occasionally send friends to.
Other than that, if somebody pings me with a position that's *appropriate*, I'll generally send back a quick "Thanks, but not now". If it seems to be a mass cold-call email as this one appeared to be, who clearly hasn't done any research to speak of, I just ignore it...
That's what I do. :)