I hate to answer a question with another question, but why did you open the door?
If you don’t or can’t afford to give out candy at Halloween then don’t turn on your porch light and don’t open the door. Both of these things make people assume that you have candy to give to trick or treaters. Did they say “trick or treat” when they knocked? If the answer is “yes” then once again, why did you open the door?
I don’t know what or if they’ll do anything at all beyond not knocking on your door next year. I would strongly suggest turning on your porch light(s) next year on Devil’s Night though, the night before Halloween. You might get toilet papered or egged.