Ask them to read you a scripture.
Perhaps Matthew 23:8-12. Where Jesus talks about religious titles. Ask them what it means.
You'll see excitement in their eyes. They know you've picked a scripture for which they are ready.
They should (if they know their stuff) explain that yes indeed religious titles are wrong and they don't use them for that very purpose. They'll be all excited to show you how they follow the bible and others don't.
Then tell them how much that bugs you and how right they are!! Tell them that just the other day two mormons came by and called themselves "missionaries" and they have "elders" in their congregations!! Can you believe that!! How anti-biblical.
Follow up before they comment with: 'But I'm glad you don't have titles like that. It's encouraging to see a religion follow the Bible. Maybe I will come to one of your meetings and see that everyone is the same, equal, no missionaries or elders or fathers or reverends or anything. Just like first century christianity."
They'll either bow out at that point or if you have a particularly hardy dub they'll point you to some scriptures where Paul appointed some as elders in the congregations.
If that happens, look at them straight in the eyes and say "are you telling me the Bible contradicts itself!!" like you're all upset with them for implying that.
Then say something like "you aren't trying to tell me that you do have titles are you?"
Finish with some righteous indignation like 'I was excited to see you follow the bible but now I realize you're just like everyone else' and close your door.
I dunno, it seems long when written but in practice it goes fast and their facial expressions are priceless.
