The "51%" bit doesn't matter. Even if you have 99.99% of the mining power of the entire network you can't steal anyone's bitcoins, you can only reverse transact

The "51%" bit doesn't matter. Even if you have 99.99% of the mining power of the entire network you can't steal anyone's bitcoins, you can only reverse transactions. This is because bitcoins are secured by public key cryptography. However, what certainly can happen, is that the community can come together and say that any bitcoins sent to the FBI's address are considered worthless. If nearly all Bitcoin users install such a patch (even if "nearly all" includes exactly zero miners), then those bitcoins will become worthless, since the value of a bitcoin is backed by nothing more than community consensus. In practice, however, such measures have been debated and universally rejected in other circumstances, because they would lead to political issues (who controls the blacklist? What stops the blacklist operators from using it to delete the funds of tax evaders, drug users, Wikileaks or whoever happens to be politically unpopular at the time?) that, once "resolved", would basically lead us right back to where the mainstream financial system stands today.