flat-earther

noun2

1. •
   A person who believes or advocates the claim that the planet Earth is flat.

   • His commitment to remaining a flat-earther seems to be strengthened by counterarguments rather than weakened by them.
2. •
   (disapproving, figurative) A person who believes or advocates an outlandish, discredited theory; a person who refuses to acknowledge the truth despite overwhelming evidence.

   • When it comes to vaccine safety, they are diehard flat-earthers: the microchips and mercury are articles of faith.