You don't need to. By ignoring or laughing at their juvenile antics, and by not engaging them as adults, you win. Not to defend Scientology, but as absurd as the gathering was it is not going to impact much of anything. The world is a bigger place than internet memes.
Such absurdity garners undue attention, which also highlights the criticism (of Scientology) they are supporting. And what does Scientology do to respond? Call them trolls, they look petty. Take them seriously, they look silly. Confront them, they look violent. Ignore them, they look like they have something to hide. And all the while the camera is rolling.
So it is a two pronged sword; you're drawing attention to the cause. And there is simply no response your opponent is able to give that benefits them.
More seriously, you raise a good point: there's no response that's good for them. Not responding is probably the best, but even it's not a great option.
Tom Cruise seems to have a sense of humor at least. Tropic Thunder made me chuckle.
I live around the corner from a Scientology chapter and often see a motley crew of kids standing across the street from the place picketing. Their appearance, their slogans, and their actions don't seem to invite what they're looking to happen, which is either to engage/educate passers-by or enrage Scientology.
Their behavior only seems to say "ignore us, we're just some loons with nothing better to do on a Saturday afternoon than to wear some cliched Guy Fawkes masks and carry posters with internet slogans."
I have to disagree. You would win if your position was not absurd. But if you, like Scientology, or for that matter politics and most religion are yourself preaching something quite absurd, then ignoring them is not an effective defense.
If you are a naked king, and there's a crowd that keeps shouting LOL, you gots no clothes on, they are winning.
The point, as ErrantX replied, is that they are being deliberately absurd, and this is a powerful one, as it calls out the absurdity in Scientology that is not deliberate. This is likely too subtle a point to actually impact anything, sadly.
It was figured out long ago. Yet those newer to the internet still don't understand, and even us hold hats slip up every now and then...