There is no way to actively filter the bitcoin blockchain.
Not without seriously destroying its usefulness.

A picture can be stored as a poem. Are you going to ban all words?

Anyway, the most likely way for you to get kiddy porn is to go to a honey pot website, and it will download a bunch of things into your cache folder, that you never even knew was there. A kiddy porn picture could be this period. And you would never know it.

The feds will do any dirty thing to bust you. Having everything squeaky clean just makes it a little harder for them. "He had a gun"

So, the best way is to ignore it.
And keep spreading bitcoin. Because the faster bitcoin spreads, the faster the FED will be dead. And they won't have any money to send thugs with guns to your house.