Sometimes they get defensive, but contradicting them and providing them with solid evidence almost never gives them "more fuel". It might get them all riled up for no reason, but that's about the worst thing that will happen if you stay calm.
An anecdote:
I once sat next to a hippie girl on a flight.
She kept talking about metaphysical crap like spirits, "energy", and karma.
When I made it clear I didn't buy any of it, she wondered why I was such a skeptic. I told her "because I have no evidence to believe in anything supernatural."
She sounded pretty convinced that she
did have evidence to believe in the supernatural. She gave her recounting of what sounded like
Young's interference experiment, but her version had a twist.
She thought that when humans watched the room, we saw the double-slit pattern. When only recording devices watched, we saw the single-slit pattern.
I chuckled and calmly explained "That's not really what happened in that experiment."
She was incredulous. We made a bet to check Wikipedia after we got off the plane, and that's what we did.
We landed, and found a place to sit down. I found the article, gave her my laptop, and let her read.
"
Oh my god, you're right!", she said. Of course... no good nerd is without a good (superficial, at least) understanding of the double-slit experiment. "This changes
everything!"
Yep. Her entire belief in the spiritual realm rested on this fabricated version of the double-slit experiment. I don't keep in touch with her, but I'll always remember that as the day that arguing with a kook actually did something.