I strongly agree with Paul here, and I would like to add a phrase that someone taught me recently:
I am not saying we should not defend or explain our faith. I am saying it is pointless to do it with people who are only trying to argue for arguments sake.
"You can't rationalize with the irrational."
I love that phrase, and I think it bears a lot of truth. If we could understand the truth behind that phrase, we would avoid many useless arguments.
So what do we do when people are being irrational? Pray for them.