Doing this (well not punching people, just saying "not now" and putting the music back on :)) I think it works because there is such a need for programmers. Otherwise, I would probably come across as a candidate with terrible communication skills.

I do that, but I always follow it with "sorry I was in the zone", which everyone instinctively gets and understands.

Higlight the was part strong enough, and maybe you'll make them feel a bit of healthy guilt about interrupting you.

I like this idea -- headphones on, head down; get into the habit of having extreme reactions to people tapping on your shoulder by flailing around and acting super surprised they were there. Should hopefully keep the random drop-ins to a minimum.