In the past, when anyone would show up without a costume, I'd still give them candy, but they wouldn't get the good candy. Instead, I'd give them that cheap bubble gum that you have to have to chew on for five minutes before it isn't like gnawing on a rock, and then it would lose its flavor 60 seconds later. I think your solution is more diplomatic and kind-hearted than mine.
I like this approach! Go out now and get some cheap Bazooka bubblegum and throw it on a window sill so it bakes in the sun and dries out 'till next year. With about a month to go, stuff it in a box with some cedar shavings, moth balls, and(!) / or those horrible wint-o-green mints, so it picks up a smell and aftertaste of that!
No costume = one single piece of dried up, smelly Bazooka. "You want good candy, wear a good costume!"
Generally lucky here too... no real old teens and no one without a costume. About the two biggest 'incidents' I've had:
A couple years ago, I was letting kids select the full size candy bar they wanted from my bowl. One little girl thought she'd be sneaky and try to palm two at once. She made her move - which was really strange because she grabbed the candy bars between her palm and thumb - leaving her four fingers sticking out, so clearly apparent something was 'weird'. She stuffed them in her bag then looked up and I was immediately looking her right back in the eye. You could see her soul melt as she thought she was going to get called out, but I just shook my head once and carried on.
Then this year - same thing - letting kids pick their candy. This girl takes a candy bar, starts to walk away and I notice something going on as she gets back to her mom. In a few seconds mom grabs the candy bar, comes up, tosses it back in my bowl. I asked if something was wrong and mom sort of snaps at me, "She's ALLERGIC to nuts!" - like I forced the candy on her and should have known better! Just sort of shaking my head in disbelief - I said, "Well, would you like a Hershey's bar, that wouldn't have nuts?" So she snaps up one of those and walks away. Wow - I let her kid choose and somehow I was the bad guy!
Then there are always the few who 'want to take an extra candy bar for some friend/sibling/cousin, etc who just couldn't make it tonight.' Yeah, OK. Hummmm...maybe I need to go get some bazooka for next year! lol!