RE: protecting an innocent person: any lawyers here?
I have trouble believing that you could get into legal trouble for having illegal incidental data from legitimate activities on your computer. If you can, then we're all in trouble, because it is very easy to put illegal data in your web browser's cache (JPEG and other image file formats let you store arbitrary, not-normally-shown metadata, for example). I don't think it would be hard to convince a jury that the block-chain is like your web browser's cache-- full of lots of incidental stuff that is needed for the system to work, but doesn't have anything to do with you. Now if you happen to have one of the private keys involved in the illegal transactions, THEN you should go directly to jail....
Adding code to "shun" certain spend-able transactions wouldn't be hard, although I think that's a bad idea for the same reason it is a bad idea to respond to trolls on forums-- you'd just encourage the bad guys by drawing attention to their misbehavior.