The answer is a definite - "it depends". Some cases are clear cut and make your decision easy. If the screen saver or other display has the potential to sexually offend someone, you must act to prevent sexual harassment through a hostile environment. Likewise, a Nazi flag is bound to offend just about everyone, and most employers would step in quickly to request that it be removed. But other areas get into the dangerous territory of "free speech". Come down too hard, and you could be accused of suppressing free speech. Not do anything, and an offended employee could try to claim you have refused to correct a hostile environment. Examples that are less clear abound, such as: religious messages or symbols, confederate flags, statements regarding unions, political messages, or macabre or gruesome images.
A good place to start is your policy manual. You should consider having a statement in there that says that personal messages or discussions that interfere with work or create a hostile environment are unacceptable If a case does arise you have to weigh the impact on offended employees against the free speech rights of the offender. You could suggest to that person that the (e.g.; screen saver) is offensive to others and request that they take it down. But if they refuse to cooperate over a borderline situation it would be wise to get a legal opinion before disciplining someone over the issue.
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