Usually laugh a bit.. and explain that it's the equivalent of me threatening them with the actions of voldemort, or the orc horde.. I then usually explain, that if their god is real.. I wouldn't want to spend any time with the narcissistic mass murder, and will enjoy the company of any alternative.
By my belief system which is to use facts and not myths I am pretty certain that no such place does exists. Furthermore , I find that a religion that is supposed to be all forgiving would condemn people who have differing beliefs to hell completely nonsense.
I try to be respectful as long as they are. I tell them that I don't believe in their afterlife and its my concern not theirs. If they get belligerent, I try to separate myself from them and if I can't I give them the old "its your hell, you burn in it".