Turn off all the lights, close all the blinds, hide in a back room and leave a note on the door saying you're sorry, but an emergency came up and you didn't have time to call them to tell them not to come!
OR
you could answer the door with a big box of kleenix, and sniffle and sneeze a little bit, tell them you think you are coming down with something, hope it's not contagious, bet they won't stay long.