[ZeroCoin]

re: "is there a deterministic way[?]"

I would assume so.

Perhaps adding a word-checker to bitcoin clients that looks for recognizable words (or attempts to validate a URL found inside the most recent block).

If block contains words or URLs, disregard it. If not, accept it.

Now Namecoin on the other hand...

[tlrobinson]

If I understand correctly, these people are using special software to encode data in transactions, namely the output addresses. You can only embed arbitrary data in blocks if you're able to mine a block (like Satoshi's headline in the genesis block and Dan Kaminsky's Ben Bernake ASCII art), otherwise you need to encode it in the transaction data.

[rlpb]

Anyone sending Bitcoin can add arbitrary data in his transaction; no need to be a miner: https://en.bitcoin.it/wiki/Script#Transaction_with_a_message

[ZeroCoin]

Well then all of the worries about this child-porn being in the blockchain are warrantless.

You could create a link to child-porn from this very comment I just posted using the right cipher. I am sure of it.

[gizmo686]

What if A) your comment was preserved as a part of the block-chain that could not be removed without doing serious damage to bitcoin. And B) the cipher you are using is a publicly known cipher recognized as the standard way of encoding data into the block-chain.

[rmc]

That almost certainly cannot work. You'll have false negatives, people will sneak the words of synonyms in (just think of how many web forums has people saying "fcuk", or "f*ck"), and false positives, legitimate transactions being banned. Will anyone from scunthorpe be able to use it?