I have had similar experiences to others who have answered, but I have a couple weird stories I'd like to share. I apologize ahead of time if this post is rambling and incoherent, it is almost 5 AM and I should be asleep.
So, first story: I was dealing with a writer I was unfamiliar with, but we seemed to hit it off immediately. We were writing constantly over the course of about two weeks, and in that brief period of time I realized that this person, while well-meaning, thought they knew what was best for my character. At first I was receptive to their suggestions OOC, but when we would actually write IC, I noticed they would make it so I basically had no choice but to go along with what they wanted. As a result, my character was changed in ways I did not like. I stopped writing with this person, but the damage had been done. Trying to reverse the changes only led to further changes, and at that point I gave up on the character. Well, I retired her, actually - wrote that she settled down, got her dream job, and had a family. She still appears as an NPC every now and then, so it isn't as if this experience totally ruined her for me.
This second story is something I've actually experienced more than once, but this particular instance is the one that stands out to me the most. It's when another writer's character dislikes yours so intensely that the story cannot progress, and the back and forth of insults, refusals, rejections, "the reason why you suck" speeches, and general degradation sours your opinion of your character to the point where you almost hate them.
So, the story: I am good friends with this one writer OOC, but we have not written together much. One of the reasons I'm hesitant to write with him now, is because I picked up on the fact that his characters were often... well, to put it bluntly, his characters tended to act like little arseholes toward mine. He writes primarily villains, and that very well, but in this case his character was not supposed to be a villain, it was just a random spacer. Anyway, this one particular thread we did was our characters meeting when they both responded to a distress signal. Over the course of the thread, his guy made my guy look like an idiot every chance he got (including manipulating the situation, i.e. they stumble upon a door that has no obvious means of opening it, my guy cuts through it with his lightsaber, then his guy goes "I could've just sliced it to unlock it, you didn't have to do that, idiot!" when there was no mention of there even being a lock mechanism, just a sealed door), refused to cooperate, ignored my guy's offers to help him, complained about how he did things during the mission (without offering an alternative beforehand, just whining about it after the fact), and generally acted like a complete prick. It got to the point where the thread was abandoned because we literally could not make progress with the storyline anymore.
It was an incredibly frustrating experience, especially because I knew this guy and considered him a friend - but my mentality of "if we're friends, why can't our characters be friends?" was a mistake. I should not have expected that, and I probably should have cut things off as soon as I began to get frustrated, rather than trying to fix it. There's no shame in leaving a thread unfinished for any reason, and doggedly pursuing some kind of marginally satisfying ending for a story that already sucks is unlikely to get you anywhere. But then so is allowing something to get to you so much that you abandon your character, so... moral of the story, I guess, is to stand up for yourself. RP writing isn't like writing fanfiction, it's a game and people are going to compete. Just keep that in mind and be aware of the motives and goals of your fellow writers. If what you want clashes with what they want, you probably shouldn't be writing together. This doesn't reflect poorly on either of you as people, it's just the rules of the game.