How do you deal with gullible people?
It depends on who they are. When I hear my mother very earnestly insisting in a conversation with my aunt that Ouija boards are dangerous, I just roll my eyes and walk away. When my daughter was 11 and insisted that everything in the news has to be true I bought her a National Enquirer to read.
If the gullible person insists on preaching, I point out that there is more evidence that bigfoot exists than there is that gods do.