There is a spot behind a building near my house where kids wearing mostly black and looking depressed hang out. One of them thought it would be clever to scrawl “There is no god” in this dramatic black script on the wall. Every time I see it, I want to add something, but I can’t think of anything witty/clever/silly/insulting to kinda spoil their hard earned angst. Any ideas?
“But there is a tooth fairy!”
I don’t know if you want something off the actual God subject, but this always makes me laugh:
God is dead.
-----Nietzsche
Nietzsche is dead.
------God
Just above and to the left, write
'The fool hath said in his heart:
and just below and to the right:
’ – Psalm 14:1
There is no goddam black paint left
Reminds me of the dyslexic agnostic insomniac.
Lied awake at night wondering if there is a dog.
“There is no God”
-
“which is too bad because he might’ve saved you when I get hold of you.”
-
“But there are morons, apparently.”
-
"Thank you for sharing. :rolleyes: "
That’s a good one! 
Unfortunately, I think that a simple “NO U” is currently in fasion.
Consider putting up nothing. It’s graffiti, and really does nothing but make the world a slightly less desirable place. Let it stand alone as a testament to a moron. (This coming from a hard-core atheist.)
But if you really feel the need to deface the world, try this:
"That’s OK. I don’t believe in you either.
-God"
In atheist Holland, the Christians Against Cursing always pay for large billboards sporting a parrot with the caption: “Cursing is learnt behavior. Don’t be a parrot!”
Invariably, someone will have scribbled: “So is prayer.”
As for the OP, I’d suggest:
“Oh, has he spoken to you?”
“He told me otherwise”.
“…so, we gonna take the blame for this ourselves, then?”
OH! Very good! 
There is no spoon.
I really can’t think of anything better than TheLoadedDog’s excellent ‘black paint’ joke. The following addition would be too lengthy to add, I’m sure; and is somewhat obscure these days anyway:
There is no God
That need be feared
So don’t be scared
To shave your beard
–Burma-Shave 19:27
Honestly, I think it’s neat that the graffiti in your neighborhood has at least a rudimentary philosophical element to it. Don’t be too hard on the kids! At least they’re taking their angst out on God, instead of the far more typical “DIE FAGGOT” sort of inscriptions. Anyway, sooner or later some rapier wit will rebut their argument with Descartes’ famous insight: “FUCK YOU!!!”
Quisquis amat, veniat.
Veneri volo frangere costas fustibus
et lumbos debilitare deae.
Si potest illa mihi tenerum pertundere pectus
quit ego non possim caput illae frangere fuste?
There is no god…
- he’s on holiday (joke)
- but Allah (controversial)
- but there is a God (grammatical)
- of graffiti (joke)
- but pray just in case (philosophical)
“so we only have each other to revere”
(or bum smokes off of)
Before you actually paint anything amend that graffiti, check with your local law enforcement types. AIUI some jurisdictions in the US have started some draconian anti-graffiti laws, and if that’s the case, I don’t think you want to risk getting on the wrong side of them for what’s basically pointless one upsmanship.
If you are willing to risk whatever consequences your locality has for the crime, I think that The Loaded Dog’s suggestion is the best, with Der Trihs’s “Thank you for sharing,” a distant second.
Win!
Yes, the Loaded Dog wins, but here’s my suggestion anyway:
“…and I say that religiously.”