We writers have websites, blogs, and message boards that warn us about the scam lit agents, for example, "Writer Beware". Do agents have the same sorts of blogs and message boards about some of the unpleasant writers who query them? Sort of "Agents Beware"?
We're browsing the same internet as you are, so no. The answer is no. Plus, it would be sort of rude of us to start branding people over the internet, if not downright immoral. No one's trying to scam us - it's not as if they don't have a manuscript when they say they do and then ask us to buy it sight-unseen (and we don't buy manuscripts anyway). As for query letters that are just plain obnoxious or annoying, those are instant rejects. End of story. Occasionally, in a shared office situation, one assistant will turn to the other and say, "Did you see this?" if everyone obviously got the same letter in the same mail bag that day. "Oh yeah, that was terrible." Laugh, move on.
There are annoying writers out there, but they're not scam artists, and sometimes, they're well-intentioned. If they're not clients and they're already acting annoying (calling a lot to check on their manuscript, etc), they don't become clients. There are essentially three kinds of clients that are annoying.
1. The bestseller who thinks too much of himself. This guy will call us to complain about how he didn't like the fruit on his party platter at the post-signing launch dinner for his third book. There's not much we can do, but this guy is usually the one who is responsible for 99% of the agent's income for the year, so we put up with him.
2. The client who doesn't know how to edit. Speaking for a moment as a writer myself, editing is really, really, really hard, so I don't blame anyone for this, but it often gets to the point of severe frustration. If this is the client's first book, we probably accepted the manuscript because we loved it but said, "It has a few flaws; do you agree to do some editing?" and the writer of course says yes, because they want to be a client. So we send them a list of issues - opener is slow, this thing doesn't make sense, character's name is spelled inconsistently, etc - and they do a host of things. They don't respond. They do respond, but change nothing. They respond and change everything, so that the manuscript looks nothing like the one we liked. Editing is tricky. You can do too much. If we get too frustrated, we may end up telling the would-be client we just can't sell their manuscript, because they can't seem to get it into enough shape to be sell-able (we leave out that last part). We've lost a lot of time, and time is money, so it's a bad situation all around.
There's also the real client, the one who's already sold a book with us, and the second or third book has serious problems that the first book didn't have. The first book they worked on for years; the second they wrote in about 6 months, and you can see the results. Said client doesn't understand why suddenly we're so critical of their work, and has the same editorial problems that writers go through. Solving problems in stories is hard, especially when the problems are structure-related or involve a major alteration of character or plot. Some writers aren't up to the task. They had one book in them, and we sold it already.
3. The client who doesn't understand deadlines. It may seem like we live in a nebulous world where we take our time getting back to you, but that's because we're rushing to meet real deadlines, ones set by publishers and editors by way of contract. We need those revisions, and we need them now. No, seriously, Penguin Putnam will not be happy if it's not on their desks by 9 am on Monday morning. They might even not pay you because of a breach of contract. Get out of that fucking restaurant, go home, finish your revisions, and send us a copy. What? You hand-wrote them?!? All right, overnight them. Yes, from England. Well, I don't care, find a post office that's open; maybe you shouldn't have put this off for 6 months! Or something like that. It's very stressful when it happens, usually because big money is involved. And there's no website to warn us about that.