Everyone else is convinced. They can't be reasoned with. So the best way to convince them is to ignore them -- eventually they'll get bored and stop obsessing over it, though it could take years or even decades. Best way to piss them off, also, is to ignore them. The one thing the Truth Movement has always thrived on is drama.
If you don't believe me, just scan through the Forums and see how often Truthers try to reply to me, knowing full well I have them irrevocably on Ignore and will never, ever respond. If their messages weren't quoted by other posters so frequently, I'd hardly know they exist -- just like 99% of the real world.
Ignoring them works. Give it a try.