The best thing to do with internet trolls is to ignore them.
In fact, that's the only thing to do with them. They just waste your time.
But it can be emotionally hard to do so, if someone's sucked you into a series of conversations and, over time, has proven to be uninterested in reasoned discussion. Believe me, I get caught like everyone else and it's hard to let go.
To make it easier for me to let go and move on to productive discussions, I wrote the following shutoff notice; you may feel free to copy and use!
Dear Mr. Troll:
This is your FINAL SHUTOFF NOTICE. You have overdrawn your account of goodwill.
It is patently obvious that you are interested only in arguing, not in discussing reasonably. You have been given many opportunities to reform, but it has proven a waste of resources to respond to anything you post.
Thank you for your attention to this. I won't be responding, because life is too short to give any to trolls.