First they describe themselves as a repressed minority under threat from "The Other". Next they make outrageous claims, and of course reasonable people will refute these claims as untrue. Then they point to these "attacks" on their belief as confirmation they are being repressed. There is also the thrill of righteous indignation, "knowing" they are right and standing up to The Other.
Arguing directly does not help, but only confirms their bias and feeds their indignation. Sometimes, with much patience, you can catch them contradicting themselves and point this out. I've learned to do this with certain Christians I disagree with, but Christianity has a central belief (the Resurrection) which all understand. Not so with conspiracies, they don't have any clear definition or fixed beliefs you can pin down. Instead you need to look for Common Sense arguments. ie: If what they believe is true there should be consequences; where are the consequences?
(If vaccines are so harmful, then why don't we see more problems among the millions of vaccinated people? Is it even possible to put microchips into a vaccine? How does that work? If they can put microchips in a vaccine then why not in our food? Avoid the vaccine won't help because they are probably chipped already!)
DON'T argue science with them, at least not at first. Draw them out and ask questions, looking for contradictions. Make them defend their own beliefs.
And one more time for emphasis:
MAKE THEM DEFEND THEIR OWN BELIEFS
