What are some good comebacks to the threat "trick or treat!"?

“Eat dog meat?”

“Have you accepted Jesus as your Lord and Saviour?”

Get lost, or lose an eye.

Beat it, or burn.

Carrot stick or spinach beet? (Have a big bowl of either ready.)

Trick or treat, you say? Hmm well now, little one…let me SPLAINS sumthin’ to your ass. Some day man, you’re gonna have to smarten the FUCK UP. Some day my little man, you’re gonna have more to worry about than just getting your damn homework done up on time. Some damn day brodo, you’ll have to get your mom’s tits outta your fuckin mouth and wake up AND SMELL THE COFFEE!!! Smeeeeeeell WUT DA ROCK IS COOKIN!!!

“sticks out tongue and goes blblblblblblbl”

You’ll grow OLD and have to get a job! Pay taxes! Fall in love, MAKE BABIES and teach them Sarah, teach them never to go to the grocery store, never to buy pumpkins and carve them into heads as if you were some kind of SICK VUGGEN NECROPHELIAC!
You’ll know then, you will REALIZE you snot nosed little savage, that you can’t just go door to door like some beggar dressed like a jackass, DEMANDING that ah give up mah shit to yo ASS!
Do you understand, you little shit!? DO YOU COMPREHEND WHAT I’M SAYIN’, FOOL?! Grow up, grow some fuckin testicles and stop being a Satanist!

NOW GET THE HELL OFF MY PORCH ‘FORE YOUR MUG ENDS UP ON MILK CARTONS!

Little bastard.

“Smell my feet.”

Actually, I love it when kids come to my door on Halloween, so my response is candy and a warm smile.

I once answered “Trick” and the kid started to tear up and began to cry. I felt terrible.

When I was a kid Halloween was one of the best nights of the year.
I remember the magic and try to pass it on.

The only proper response should be: “Happy Halloween! Here’s your candy!”
If you must say something else you can add “Nice Costume!”

How about a cocking shotgun sound followed by a sound clip of clint eastwood saying “get off my lawn”?

Just say “trick” and offer them an ice cube instead of a treat.