That's my general policy too, but I'm a pushover. I can't just tell someone "no candy for you." So if a teen comes up without a costume, I'll ask them what they're going as. If they can't come up with anything clever (like "a 2014 guy!" "my friend chelsea!" "a regular person!" "a serial killer - they look like everyone else!") and just say themselves, then I ask them if they're at least a really interesting person worthy of being dressed as. If they can't dress up, they've at least got to banter with me for a minute for their candy.