Tell him/her to logically defend his/her comments.
For example, when an Atheist calls a believers fools and informs you that "there is no evidence for God", tell the atheist to show proof that there is no evidence.
If an Atheist makes a statement like the one I just posted, the Atheist should be ready to present data supporting the argument.
A truly ignorant person will holler back you: "You can't prove a negative!" Ask that person how they know for sure that no evidence exists.
What I find to be a common thread among anti-theistic antagonists is that they think they have all the answers and they're not compelled to support them with facts.
Or, in some cases, they'll just throw links to websites like evilbible or jesusneverexisted.
I have found that most Atheists on the internet want to lecture. They seldom want to discuss. When you press them to logically support their opinions, and you point out their illogic, they get nasty and eventually go ballistic.
Remember, these guys are supposed to be the adults in the room.
Remind them to act like adults.