Well, it just shows you can find almost anything on the internet, because the answer to your question can be found right here:

If you don't have time to watch the video (though it is quite short), the basic answer is that you push the microphone against your nose and use the friction to scratch it (an astronaut would always have a microphone on a spacewalk, so no problem there). And of course if you are inside the spacecraft you can take your helmet off, so that's all right.

As for an itch anywhere else, you could give a general rub to most places, which would relieve the feeling somewhat. I think an itchy foot would be more of a problem - but of course it would be the same as working in very cold conditions, for example, when you would also be wearing thick clothes and boots that you couldn't take off.