Of course, most of the time this type of request comes from management that neither has a clue about what the users really want, nor does [s]he have a clue about the technical aspects of building a specific software project or software in general. See Dilbert's Pointy-Haired Boss for more detail.
However, that's just one aspect. What about requests for items that you know will hurt overall performance of the system you're building? Or how about the technical idiot who's foolishly been given authority and yet nearly everything they do turns to dook? (See this post for an awesome example)
Ultimately, how do you eloquently, professionally and gently deal with requests or edicts to what you're building that you know will ultimately hurt the project?
Dupe: When the Client asks for something ludicrous and insists