I've gotten into those disCUSSions too and when they play the "you'll go to hell" card, I just respond, "No I won't because I don't believe in it. Besides, as the bible says No one know the mind of god. So you aren't in any position to determine who go to hell and who goes to heaven".
Then I usually walk away, if they walk with me and persist I remain quiet and keep walking away.
If I am in a devilish mood (pun intended), I'll just roll my eyes, go "awww geeze", give a contemptuous laugh, shake my head and then walk away.