This is more of a general etiquette question about what to do in this situation, but I will use a particular situation as an example.
Andrew has a really obvious 'thing' for Rachel. They're both single. Rachel isn't interested in Andrew, and she's aware of his interest. He definitely thinks he is being subtle, but is really not. He's not being creepy, but it is quite plain what's going on. He has that 'look' in his eye. He makes jokes that are really not jokes, flirts awkwardly, and has told many people that's he's interested in her. He's never made a direct move on her or asked her out, either because he's getting up the nerve or is too scared. He is a really nice guy, but Rachel is not attracted to him. They see each other pretty frequently as they have a lot of mutual friends, share interests, and go to the same weekly activities.
What should Rachel do? Going up to him and saying, "I'm not interested" seems really harsh and awkward, but it's also awkward to let it continue. Subtle messages like making sure not to sit near him are not being noticed. Or maybe he does know she probably isn't interested, and isn't aware he's being obvious. Would it be acceptable for Rachel to have one of Andrew's friends tell him 'this is really obvious, stop'? That seems childish too.
I used a specific situation but I'm also very interested in thoughts on the general. Would your answer change if it was Andrea really interested in Robert? Also, if possible, I would like to avoid talking about what to do if someone is feeling stalked, creeped out, or unsafe, because that is a totally different situation. This is specifically about when someone is pleasant, maybe even a friend, but has an obvious infatuation/interest that is not reciprocated.