Free reign on a conversation doesn't happen often in life, but when (illegal) promotional caller/credit card scammers call me, it's fun to try to draw them into bizarre conversation... brief example being "you know, it's not the best time to talk... I've just dismembered a hooker, and I'm trying to figure out recipes..."
This might not be fully no-consequences conversation, but I'd still suggest:
1) fake a seizure. You don't have to respond to anything anymore (remember to bite your tongue and wet yourself; don't verbalize during the seizure, and act very sleepy afterwards, this will make it all seem very real)
2)Yes, offer to take a picture, as above, but aim it directly at their chest from point blank and fire high speed burst. Then ask if you can have a hair sample, or sock or something personal from them.
In reality I usually just say thanks... or if it's somebody I sorta know and I'm with the big lenses I'll just mention that I have a lot that I need to compensate for, that will quickly defuse the situation. Or just say that I really enjoy taking pictures of my kids. Frustratingly, these responses are not as funny as the other 2 options.