Addressing the OP:
your best bet is to blink, nod, and smile. Do NOT actually engage them in discussing their theories: that is a rabbit hole that goes on forever.
Or, if you feel like being somewhat perverse: research such theories, find the most ridiculous, and verbally embrace it when your coworkers start up with theirs'. Believe me: pretty much every conspiracy theory can be tied to every other, often with humorous results...