I think whether or not you should attempt to debunk conspiracy theories depends on your goals. It would definitely be entertaining, and if you delve deep into them you just might find a kernel of truth or something interesting like that. And like you said, if absurd notions are not challenged enough, they just might gain enough traction to cause something bad (see: Pizzagate).

Trying to reason with the hardcore nutjobs is certainly a lost cause, though.