Thanks for being here with me!

I laugh and ask them where is this place called Hell? Also suggest it's perhaps a state of mind rather than a place and ask if they can relate to that?

I simply say, ok.

As a Satanist (unfortunately many people judge, misunderstand what that means) I don't believe in a Heaven or a Hell, and I would just chuckle quietly, look the person in the eye, and walk away

Sounds more interesting than Heaven anyway.

I had a badly autistic child and was told it was my punishment for being sinful! The cruelty was awful!

I always say “good, that’s where all the awesome people will be.”

I tell them that if hell exists I probably belong there anyway so bring it on!

I find their disconnect to be rather curious. How do I even know they're working for the "good" side?

I just laugh and walk away.

this would depend on whom was making the threat, but generally I would simply ignore it or make a sarcastic comment in return

If there is a God and he doesn't understand why I don't believe than he is not the God they talk about.

I sometime am very sarcastic and other times I just let it go because I see they are fragile and could be hurt easily by my sarcasm.

Laughter... at least on the inside.

If I don't believe in your vengeful god how can I fear him?

Well, something had to have prompted that to cause them to say such things. Depending upon what started it, my response may range from laughter to engaging them in argument over their ideas.

The religion I belonged to didn't believe in hell, however they did believe God would kill non-believers during Armageddon. To that I would say, I'd rather die with the other billions of people in Armageddon than live in "paradise" worshiping a God who would do that.

Incredulity!

To be honest, when threatened with punishment and / or damnation, I simply tell the person damning me that I respect their right to have that belief, but I do not believe it will happen, and to leave it at that.

Has not happened, yet.
But I'd be inclined to tell them that saying that is naughty, and so Santa won't bring them presents.
And try to explain that threats of Hell make as much sense to me as my Santa threat does to them.

I simply tell them to see help for their mental problem.

I laugh.

Possibly comment on how it'll be a party and that with all the other scientists we'll have probably figured out some geothermal powered cooling system. If they persist then I'll pull out my notebook and start explaining science to them until they get bored/frustrated and walk away.

I ussuallytry to just walk away from them, you can not argue with a fanatic, its not worth it.

Laughter is always appropriate.

Why would an all loving caring God want to punish the people he loves.