Megan Collins, of the Style Girlfriend blog, 
had a rather disturbing experience in an elevator recently, and was kind enough to share it with the internet. It involved a rather horrid chap who goes around making rude noises and staring at women's breasts in elevators, small public metal boxes with no hope of rapid escape. Do read her post, she is being unbelievably polite about the whole incident, but I don't think people like this deserve as much patience, and can only say:
This man was being hideously disrespectful. In any public space, especially a small metal box, entering somebody's personal pace should be done carefully and politely, with the afterthought that this person may not want to interact with you. This has nothing to do guys and girls. This has nothing to do with style. This is simply about common courtesy.
Feel like talking to somebody in an elevator? Okay, say something, but be polite, and be prepared for an “I don't want to talk to you”. If you're being ignored, that means “I don't want to talk to you.”. An awkward grimace? Could mean the same. If they don't want to talk to you, back off.
Want to chat up that hot girl that just entered the elevator? Try polite conversation first, you misogynistic creep.
Feel like staring at somebody or making rude noises? Makes me wonder who let you go out in public.
I'm not saying anything new here, am I? To me, these simple guidelines are very obvious, and it's perfectly acceptable to hold others to them in public. So, if a stranger start talking to you and you're not in the mood for talking, say so. (on the other hand, don't be afraid of talking to strangers. They could be very interesting people) If somebody is staring at you, for goodness' sake, protest. If somebody is making rude noises, and you're in no position to walk away in obvious disgust, I should think it's perfectly okay to ask them to stop.
There's no imperative here, of course. There is absolutely nothing wrong with just politely looking the other way. However, the public sphere does not have to be the obscene person's playground. They don't deserve it.