pawsplay
Hero
If the player were interested in this kind of thing, they probably wouldn't object, and alterations over objections are what I'm talking about.
Is it ever permissible to overwrite a character's background over their objections? I'm going to say yes, in some situations. I don't recommend it if avoidable, but sometimes the GM has to make things work and the problem may not be noticed until after things have been put into play. The clone/memory wipe scenario is not something I would press in like a Conan-inspired game, but it might be an integral plot element in another kind of game. If one or more players object enough to leave the game, I guess that could be it for the scenario, but that is not really distinct from other situations where the players reject the GM's scenario and leave the game. Assuming the scnerio makes some kind of sense within the context of the game, I think it's possible for the players to object to and even be offended by the premise, with there not being much left to do other than the GM saying, "Well, this is basically what's on the menu. How do we proceed?"
I never want that to happen, but it certainly can. Some examples I can think of are some Vampire games I played a number of years ago. I was in one game, where the GM decided to juke the character creation rules and hand out Generation according to the sires she assigned us based on our backgrounds. I got stuck as a 12th gen, while another player, who happened to be her boyfiend, got 9th gen with a character concept I thought was a problem in the first place (note for non-Vampire players: 9th generation is significantly powerful for a starting character). But I decided to deal with it. When his PC got into some trouble, I and another player had our PCs assigned to him to babysit, and when he continued to do problematic things, well... at some point, we ended up staking him and leaving him in a closet overnight, for safekeeping. He was incensed at being staked, I think he was miffed that his powerful PC was one-upped through some quick thinking, etc etc. I can certainly understand his frustration. But I don't think he had a right to complain about being babysat, or the consequences that followed, because the GM was trying to deal with the situation as it developed. So basically, I and a couple of other players were dealing with being handed weaker character sheets than he, while he was dealing with being put, ultimately, in a socially inferior position, as a sort of balance. His PC was lower generation, but less experienced. Right? But it wasn't an element of his choosing and I think the way he responded was not helpful.
Similarly, if you're playing a variant supers setting, like Paragons or Wild Talents or whatever, and you're playing a wizard, you can cry all you like when it turns out your powers are psionic or mutant or whatever, but it's kind of written into the setting that the GM, if anyone, knows the ultimate truth for the setting. Whining about your powers not really being magical, or I guess conversely about all superpowers coming from magic or divine blood or whatever, is not helpful. I don't think there's a reasonable justification for complaining.
And if you're playing D&D, and you find out your character's father was not really his father, well, that might cheese you off, but since it doesn't affect your backstory as you originally wrote it, nor the actions your character takes, but effectively only future events leading from that revelation on, well, that's pretty fair game, I think. I would try to work with a player who objected to something like that, but if I'm in a corner, I would probably have to say, "Look, can you work with me here?"
If a player digs and just refuses to accept something in the game, I can sympathize, and they are within their rights to quit, and I recognize in many cases there are real personal issues that might make that necessary, but you know, that happens sometimes. It is, in the end, just a game. If you're married and you really don't want to deal with any hint of romance toward your PC, I would suggest dealing with it in IC, then trying to see if something can be worked out OOC, and if the situation is still uncomfortable.... well, don't waste any time looking for a new group with a more compatible style.
So going back to your post, it presupposes refusal. Whether I would override someone's wishes who absolutely refuses is kind of a non-question, since if they refuse, I have no ability to compel them to cooperate. They might derail the game or quit or argue others into making concessions, but my belief about whether they have a right to do so is quite theoretical at that point. Which is one reason I prefer not to deal with players who are ofen, or even more than rarely, inclined to argue about stuff that happens that makes reasonable sense within the scope of the game.