I find the best tactic is to ask why ?
Why did space lizards take over the Royals, why are NASA lying about the shape of the earth etc.
I have debated a fair few flat earthers online, and TBH, I enjoy it. They tend to latch onto things that are taken for granted as a starting point, and most people can't explain it. Then once their gas lighting is done, and you are maybe struggling to explain the taken for granted stuff, it's into the rabbit warren they go, ducking and diving down different rabbit holes every time they are challenged.
To deal with them, to have a normal conversation, I think its best to try keep the conversation out of the rabbit warren, and if they go down it, don't follow them.
Keep them on the basics. Why ?
Conspiracy theorists can be, and very often are, very knowledgeable about their specialist subject. and if you are not prepared, as most people are not, then they feel they have gained a victory. And that reinforces them.
Ohh, and ban the word " whatabout".
If they say "whatabout", just say " whataboutism: logical fallacy ".