The only way to answer the door when you do not know who the person at the door is, is to look outside BEFORE opening the door, people these days do some crazy S@#^! And you don't want to let them into your house, do you? Secondly, if you absolutely feel the need to open the door even though you have no idea who this person is, wait till a bigger, older, person gets home.
Don't make your presence aware if you have no intentions of opening the door, so just lay low, and turn off the TV and other things that make noise. They will go away sooner or later.
I never answer the door. Ever. Once my grandpa's friend or something came over to our house and I still didn't open the door, because I've always been told to not open the door EVER!! I didn't know that that's who it was so...