You know what I would do if I was the owner of this billboard. I would just go up there with a can of spray paint. Draw an arrow pointing to the "under God" part. Then I would write "added in 1953", and leave the billboard up for a couple more weeks before replacing it.
Christians tend to have this really annoying persecution complex in which they think that they're the righteous minority and that everyone else in the universe is constantly trying to stifle their religious rights. Meanwhile, back in reality, christians actually make up the overwhelming majority of America and are perpetually shoving their own beliefs down the throats of everyone else, and when someone tries to stand up for their own rights, the religious right is there to scream "PERSECUTION!!!" and bitch about how the godless, immoral liberals are communists who hate america.
I've never heard of a 'persecution complex' before, but that's exactly what they have. It's goddamn infuriating. And while it may not do any good to vandalize their property, it's what Jesus would have done. ;)