The billboard doesn't prove there is no good, but it debunks the morrons in the US which claim the Bible provides a moral standart. When you finally get these christians to admit this isn't the case, you have gone a step forward.
And when we can go to the next stage:
Why do you believe in god? Do you have any arguments? Yes? Well, we have debunked them. No? OK, then why do you still believe in it? Because atheists can't disprove it?
Here the "with a probability bordering on certainty" argument comes into play, in short, the belief in god is standing in one line with alien believers, creationists, people who think we live in the matrix and Superman really existing.
So, when this has been established, religious people are down to "well, I do believe in it, and I'm not going to change that." BANG! Step by step, the amount of believers has been taken reduced heavily. And step by step, these people will be in the same line as other people believing in lunatic crap like "the driver shot Kennedy" or "no planes hit the WTC".
Once the belief in (with a probability bordering on certainty) nonexisting gods has been almost eliminated... Now with the (very often really stupid) moral values that come with it: They are open to critique now without the possiblity to block it with "it's gods word, yadayadayada". That's it! Religion has been eliminated.
What do I hear? You will never reach that point? Probably not (at least not fast), but with each step, we will have reduced religious believes and the harm that comes from them.
And no, people who believe in bullshit are not all retarded and stupid, in fact most are not. We psychologically (is this the word I'm looking for...? You get what I mean) all believe in some kind of bullshit, often in bullshit being harmful to ourselves and others) and it is often hard to desinfect ourselves fom these viruses. It's nothing to be ashamed of, that's why tons of people go to the psychologist. Religion is only another harmful virus that has been living with us for too long. Time to get rid of it.
tl&dr:
Step by step, people. Step by step.