I think more context is needed to offer any kind of suggestion. I have friends that are theist that will say things like that. I don't take offense. I assume they have limited exposure to atheist. My goal is to break the stereotype of the Imagine in their head. But if I was at a bar and someone over heard a conversation and started jumping in with that kind of stuff... It would be a different reaction.

What will you want to do? Do you feel threatened by the diagnosis?

Bite their kid.

I don't know that it's ever been insinuated to my face that I have a mental disorder. Well, not because of my being an atheist anyways.
I've been told I am wrong and that I will come to realize that when I am burning in hell for all eternity. But, I've learned a lot of things the hard way so...

Believing only that for which there is supporting evidence is realism, not a disorder.

You always can direct then to visit the "Clergy Project"...a website for guys who have left the priesthood. Enlightening.

They don't have room to talk what with their imaginary friends and all.

I either walk away or say "Interesting, especially coming from a person who worships an imaginary friend."

laugh and then using logic explain to them that "true believers" are the ones with the mental disorder.

Any energy you'd spend would be a win for them. I've learned long ago not to engage in meaningless arguments trying to prove a point to someone that doesn't have the ability to understand. Smirk and walk away.

Shoot them and claim mental disability.

Ask them if they posess a degree in psychology and request to see the credentials which gives them the background and authority to evaluate your mental stability.

Tell them you know an entire website full of non believers , welcome them to come challenge a vast amount of very intense intelectuales.

Start worshiping Satan, because apparently it's a step up.

Whip it out and start peeing on their leg or shoes (for women). That will make them go away and leave you alone.

Atheist are open minded to religon