I totally know what you mean ...

I have never actually done the not answering thing with people I know (but have with door-to-door people a few times, since being pg and having a newborn) and have a couple of times not let people I know inside ...

Standing in the doorway and saying something along the lines of, "It's really nice of you to think of me, but I'm acutally in the middle of something at the moment/going out shortly, and can't stop and chat. But give me a bell tomorrow, and we can set a time to have lunch/coffee/dinner."

I have also on a couple of occations answered in my nightie/bathrobe/towell/the nude (not showing anything - wooden doors are wonderful thing) and let it be really clear that now is *not* a good time. "Sorry, I'm a little naked right now. But how about dinner on thursday?"

Good luck!

Similar problems can pop up whedn you give birth, and are trying to work out ways to stop people from visiting you in the hospital, when you're trying to meet your baby and learn to BF (if you choose to)...