I have an uncle like that. Can't reason with him - he's firmly convinced, to the point of swearing on his mother's grave, that this or that thing he read in a forwarded e-mail is true.

In my experience, I found it's best to let him rant and rave and either space out or follow what he's saying, but in a completely dispassionate way. Neither agreeing nor disagreeing. Agreeing will give him more fuel, disagreeing will lead to an argument in 9 out of 10 situations.