I'm thinking about some kind of civil asset forfeiture program. Issue legal orders seizing any bitcoin that are alleged to have been used in the commission of a crime. Then bitcoin holders would have to prove their specific bitcoin were not used in commission of a crime.

For small holders it wouldn't do much, since no one is declaring their bitcoin. However, the SEC could require public companies to only hold or transfer "clean" bitcoin. That would make it much less attractive to buy or hold bitcoin.