Simply being aware of this aspect of the human conditon can offset it, but only to a point. To get me the rest of the way to fairly assessing my opponents’ points, I’ve started ending debates with an admission of sorts. If I’ve learned anything new, I let them know. Not only that, I let them know in the order that they would want to hear it. For example, if they actually offered an argument that makes me think the possibility of a deity is just a little more likely then before, I’ll point that out. If they corrected a flaw in an argument I use, I’ll admit to it and assure them I will no longer use the faulty argument. If they bring to light a novel way of looking at things, even if this way doesn't change my mind about anything, I will still offer whatever praise I can for originality. And if all I can commend is my opponent’s friendly disposition, then I will do so.
Examples of general phrases to end a positive exchange with another human being:
Thanks for the conversation, you are a congenial voice for your worldview. I hope to chat with you again.
I feel better about Christianity/Islam/Hinduism/Magic: The Gathering now than I did prior to our talk.
Gosh, you're swell.You get the picture. Truth be told, I get to use these closers more often than not. Many religions pride themselves on brotherhood and non-confrontational missionary work and it shows. Your results may very, especially if you tend to engage trolls.
Still, they can't all be winners. Some people are too passionate about their faith to have a calm conversation with someone who isn't seeing things as they do. Others are going through a type of cognitive dissonance which makes them so uncomfortable they attempt to make it stop by sabotaging the debate. In poker, we call this going on tilt. A player gets mad about a bad beat and starts to play sloppy--this can quickly end a game by, most often, the tilted player losing all his money, or the tilted player gets lucky with their erratic behavior and hits big--taking the other player out of the game. The "tilted" debater can force the same two outcomes by either descending into irrational name calling or prompting a similar reaction from you. My advice? Don't return crazy with crazy. Diffuse the argument with a simple closer and move on. Atheists have a bad enough wrap as it is without more of us throwing around f-bomb laced ad hominems.
Examples of general phrases to end a negative exchange with another human being:
I understand your frustration. I'll think about what you've offered, please consider thinking about what I've offered. (this works even if you have no intention of thinking about their argument further.)
At least we can agree that one of us is right. (I like ending this with a winking emoticon.)
God bless. (Sure, you don't believe in God, but they do. This may or may not be said mockingly depending on your nature, and may or may not be taken as mocking depending on theirs.)