The most effective way to disarm anyone who is on a p roselytizing mission to prove they are right and you wrong is polite disinterest, nothing sticks in the craw of those on a mission to save the world than total disinterest, they have great difficulty in getting there infected/affected minds around polite disinterest, their very very important message is deflated to an importance level more akin to kids ringing your doorbell and playing trick or treat, simply smile and say "I'm not very interested, you do look very smart though, please call again any time" you can almost guarentee they wont.
Regards
Phil