I have purposefully stopped talking to some people I know. One such situation is in failed romantic relationships. None of mine have been by mutual agreement - on some situations I have been the one to end it, and others I would have preferred to continue. Either way this sort of power imbalance leads to a situation where the best choice seems to be strict avoidance - whether the aim is to avoid lowering one's status by appearing desperate in pursuit when the end has been made clear by the other party, or to avoid giving any possibility of signals that a continued relationship is still possible (similar to the point paper-machine already made).

But this is not the only circumstance I've stopped talking to people I know - I do the same thing for some extreme zealots, whether their cause is religious, political or (gasp) even "rational". Here's one (intentionally vague) example from my recent experience. one of my longtime friends has beome very interested in a particular movement, and now has nothing else to say on any other topic. He feels this topic is so important that everyone needs to know the details (and presumably act upon them in some way, although this point is never reached). Clearly it is a subject he feels passionate about, and he feels so well informed that he expects others to immediately update on the force of his arguments. And if they fail to do so he makes it clear that he has won the debate. Whether he is truly well informed or not, he is not being effective in producing attitude change in others, or appreciating what others are hoping to get out of a conversation. In other words, highly visible low status behaviour of failing to recognise one's audience and not negotiating a conversation acceptable to all parties. When he came out and asked me why I was avoiding him, my answer was "Because you only talk about (topic) and I don't think this is productive". Unfortunately this led to another round of haranguing, and the same old arguments about why (topic) is the most important issue and I'm a fool if I don't see it. Which didn't exactly convince me I should continue talking with him in future, I felt it would be the same issue again and again. But it can be hard to come out and say "Your topic doesn't interest me" to someone who considers that subject of ultimate importance - it can easily be construed as a personal attack rather than being directed at the topic itself.

I've rambled a while already but there are still other situations where I stop talking to someone, especially concerning calls/texts/IMs rather than face to face conversations. These are harder to define because they fall into the category of "falling out of contact", for no clear reason. Perhaps I'm too busy when a message comes in, stick it in an ineffectively managed "to reply" box and forget about it. Or perhaps I have nothing to say at that moment, etc. If that person then comes out and asks me "why aren't you replying to my messages", my initial reaction may be embarrassment and apology. But other times the "why aren't you talking to me" is presented as a challenge, as if I am intentionally doing so. I would likely respond more assertively in such cases (or avoid responding to avoid causing further unintentional offence, I am certainly imperfect in reading some social cues).

*I intentionally avoided saying what my friend's obsessive focus was about, because I think it applies to many situations - but thought I should mention that in this case it was the old classic "Jesus died for your sins and you should accept him as your personal Saviour". If this example gives you a serious UGH enough to discount my discussion above, please replace this quote with "911 was an international conspiracy", "GM crops should be banned" or "everyone should be cryopreserved upon death".