The only way, per the system's design, would be a network wide whitelist or blacklist of accounts. Either explicitly permitting (which wouldn't work) or explicitly blocking addresses from participating in transactions. And, if enough computers disregarded it, the transactions would still go into valid blocks. The blacklist/whitelist nodes would have to accept those blocks or you'd end up with a forked blockchain or serious accounting issues because some nodes would disregard certain transactions.

A court could simply order them frozen, and expect the holder to comply. If the court deems that too dangerous, they would order them transferred to a third party guardian (usually the local sheriff or similar, but they might make a different arrangement for something unusual like bitcoin). Possibly "without notice", if "dissipation" seems likely if the holder knows it's coming.

Pretty much the same as you'd do with cash or other physical assets.

(Transfer is necessary; seizing the private key isn't secure against copies. The FBI is known to have moved the Silk Road money to their own wallet: http://blockchain.info/address/1F1tAaz5x1HUXrCNLbtMDqcw6o5GN... )

Raid their place, copy their wallet, move the bitcoins to yours.

You can't, and this isn't about freezing bitcoins. They try to prevent people from freezing their non-bitcoin assets.

Hm... would it be possible to have a blacklist of bitcoin addresses and get a majority of miners to ignore transactions that involve these addresses?