A sad fact of real conspiracies is that it's remarkably easy to make its victims into total crackpots if you have money & malice.
First, threaten them to keep their silence.
If this doesn't work, hire some people for low-budget character assassination etc.
Conversation
As pressure on the victim increases & damage is done to their reputation & livelihood, have it intimated to them that some nebulous force is after them.
(Not you, of course.)
At this point, most people will have started to go a little bit insane.