In a free society, it's very difficult to prevent some offenses without making the society less free, and that cost must always be weighed.The contradiction was exactly the point; of course I don't want to lock either of you up, and nor can you execute or jail this depraved specimen of humanity because of what he may do. Furthermore, this freedom must be afforded not only to people you like, but even, nay, especially when you are emotionally involved.
What law would you create that would target him, but could not be abused to wrongly convict the innocent? It's not a trivial question. One extreme is to make it illegal to publish photographs of any child, but I don't believe society wants that. Would you make it illegal to publish photographs of any child of which you are not the guardian? How do you verify it? Or do you tie the illegality to how the image is presented (e.g. in a sexual context), also a fine line to judge (does publishing the results of a teen beauty contest cross it?) Would people have to cut other children out of pictures of their child playing team sports? Is great-aunt Bertha allowed to publish photos of her grandniece?
How does society enforce preventative measures without government? If you mean taking the law into your own hands, the bottom of that slope is anarchy. This man is not physically harming children as far as we know, so there is no "cost of a child" to wave around, and again, we do not punish people for what they might do (cf. Minority Report).