If I don't recognise the number on the caller-id, I pick up the phone and reply "Good Afternoon (Insert pizza delivery firm name of choice) pizza, may I take your order?" in the nicest, and sunniest, voice I can muster. After that you just need to stick to the script. Like: "I'm sorry sir/madam, we have no-one here by that name, would you like to order a pizza?" or if they pause in shock from the first part, get in there quickly and say "Hello? May I take your order please?"
You'll be surprised at how often the calls stop coming after that