You're screwed if you don't give them some candy.
It's once a year. Buy a couple bags of candy, and give it to them.
If you can't afford that once a year, then I don't see what you're worried about, since you likely don't have a house either.
If you're a REAL stuckup jackass with no festive spirit or appreciation of the importance of Trick or Treating, whenever kids come up to your door, just say (as nicely as possible) "I'm soo sorry, but I'm out of candy. *sad face*"
You won't be vandalized, because they think you're a decent human being with a soul. Much less troublesome than ACTUALLY having a soul, right?
I mean... Who cares about the children, anyway? They shouldn't expect to get candy once a year, that's WAY too often.
You should take it to government, and have them change Australian Halloween to only once every 10 years. Much healthier, and you won't have to spend 20 dollars a year!
But why stop there? 20 dollars every 10 years is STILL a lot of money to be giving away in the spirit of generosity and good will towards your fellow man, right? You might be able to get Halloween banned entirely!
After all, when you were a kid, you didn't have any friends to go trick or treating with, and your parents were usually too drunk to take you themselves, so you have a RIGHT to hate Halloween.
Oh, if you're reason for not wanting to give out candy is because you're busy or something, put a bowl of candy on your porch with a sign that says "Take one please! Sorry I couldn't be here... Happy Halloween!"
And put a sign under the bowl that says "If there's no bowl here, it was stolen. I gave you all candy. Please don't vandalize my home."