How do you debunk conspiracy theories effectively? Based on what I've seen over the years what works for people is a combination of finding out things they believed to be true were actually false, seeing people they used as sources of information give out wrong information, and being exposed to new information that gives them a more realistic context in which they can figure things out for themselves. So those are the things I focus on.
Some of this ad-hoc approach is validated by the study Debunking: A Meta-Analysis of the Psychological Efficacy of Messages Countering Misinformation, published in Psychological Science
The paper itself goes into a little more detail on these points:
The research brings up the backfire effect - where debunking something simply can actually reinforce that false belief. This is a real problem, but can be addressed by the overall strategy of providing new (true) information and gently encouraging people to "do their own research" in a more rigorous and reality based framework.
"Keep expectations low" is a key point. Don't get frustrated and angry when a debunking fails to take. Give it time, let it sink in, and keep lines of communication open. Effective debunking requires time for people to come around to actual reality, and in the case of those deep down the rabbit hole this time may be considerable.