@bno to add to what others are suggesting, you have to think of conspiracy as a puzzle in front of them that they need to solve and are in the process of solving. That need is emational. The conspiracy theory is really attractive because it looks like a solution. Often they chat to you because they want to test their opinions against the puzzle. The worst thing you can do is say the puzzle is not worth solving, but something really helpful is to give them a piece which is "extremely true"; something they cannot deny technically or emotionally. You'll know this because you know them as people. When you give them this puzzle piece, it's a confounding piece for conspiracy to enter because conspiracy needs a blank slate so to speak. Enough of these pieces and the conspiracy cannot hold, but the need to put pieces together is important still, so you need to have some understanding of the need to be able to fill it.

Unfortunately, you probably also believe a bunch of stuff that isn't true (eg most economics) and you need to confront that too. Solve the puzzle together.