Every Community Manager knows that a few bad apples can spoil the bunch. One difficult user can make managing a community so incredibly exhausting that there is little energy left for posting or engaging.
Conversations on communities are much like little creeks running through a forest. They run small, but clearly on their own, unless someone dumps a shopping cart load full of garbage into them and blocks up the flow.
Here are a few of the most common problems caused by problem users on any community:
1) Never Been on a Community Before - This person has also, apparently lived in a cave and never developed any social skills in the normal range. They type in all caps, post hot pink letters on black backgrounds and resist all attempts at training. Of the kinds I have had to remove from communities, these are some of the saddest, because they really don't understand why they need to turn off the caps lock, even after they've been told that they are "screaming" at the community.
1a) Never Been On-Topic - A common offshoot of the above is the person who is so EXCITED to have someone to talk to, that they talk about EVERYTHING. These people pay no attention to on-topic/off-topic. They'll post Internet memes from 1996, (cookie recipe, anyone?) and political outrage and will not stop until you moderate or suspend them. It's all so exciting and new! This fills the community with spammy, off-topic stuff which chokes out good conversation.
2) What Post, What Sentence, What Word - These people are not going to admit they are wrong or rude until you can post the exact word that offended and then they will say "There's a smile emoticon" and tell you that people need to be less uptight. In the meantime, they snark and snap their way around the conversations on the community until their presence on a thread guarantees its death. I used to get this very often when I moderated a Martial Arts ML. Many of the posters did not "see" that they were just being jerks. They insisted on knowing which word made them a jerk. The behavior was jerky in and of itself and became grounds for being banned from the list. When one person is a jerk, the women stop posting at all, because every comment becomes a petty little battle that is just-not-worth-the-agita. Good conversation gets choked, good posters start talking somewhere else.
3) You Didn't Do Your Research - This person needs to "win" every conversation, because all conversations are arguments. If you disagree with them, they will declare that you didn't do your research because (fill in obscure fact that was not relevant to argument but makes them look/feel important.) This is exhausting, but rarely grounds for removal unless it kills so many conversations that the community is dying.
4) Trolls - We all know and love trolls, but trolling is a complicated thing. It may be as simple as asking pointed questions with words seeded like mines. It may be a post with massive assumptions that could take a year to parse then refute or it maybe the subtle art of conversation drift, when they focus on a word or phrase out of context, then make assumptions based on the "real" meaning. Trolling is often simply just being negative all the time. I recently had to escort a person off-list because they simply never contributed anything, ever, of positive value to the group. The only posts they ever made were negative, nasty or exhausting. To counter this, I have begun adding "Nurture the conversation" in to my community guidelines.
5) I Disagree With What You Didn't Say - Cognitive dissonance is related to trolling, but they are not the same. With cognitive dissonance, repliers are responding to something that they imagine was meant.This will often start fights within the community about what things were said or not said...it becomes "that thread" that will eventually be closed by moderators.
6) Punish the Popular - There are a number of people on the Internet who prefer to spend time and energy trying to shoot people off pedestals, rather than contributing value of their own. When people get to a certain level of expertise, or become a nexus of information...in other words, they become popular, these people cluster around waiting for their chance to shoot these people down. The desired effect is to make popular posters feel sad and possibly drive them away. Sometimes, the dissatisfied go to other forums and set up "why we hate soandso" threads in hopes that the word gets back to them. When this gets too common on a community, the harassers can hurt the community by driving away good posters with their tactics.
Ideally, everyone on a community wants to promote, nurture and grow the community. Baking that into community standards right from the beginning will help, but it won't stop conversation being choked by a few people with or without an agenda.