First thing about ending conversations with nutters...always make calm statements about your position (not about what they are talking about).
If they keep going, repeat...and repeat..and repeat (never raise what they are talking about)
That's one angle (works with most people, not just nutters...but nutters require more patience usually)
You can also try using their religion against them :
-don't you believe in loving people as they are, not as you want them to be?
-don't you believe in free will?
Or, as they believe everyone wants to go to heaven / see their dead relatives again / live forever
"I don't want to go to heaven"
"I don't want to live forever"
"I don't want to not be married" (Jesus said there would be no marriage in heaven)
"I don't want to fornicate" (irony here - there being no marriage, and still having sex organs would present a problem for the christian faith, would it not?)
but really, the first suggestion eventually is the best - you are firm, clear, and respectful about your position...of course, you may have to use this tactic on lots of different people, which can get very old.
There are other tactics too...depends on what amuses you, how respectful you wish to be, and how persistent they are.