[rtpg]

if you could get such a thing in place, couldn't everyone just roll back transactions?

I feel like having such a system in place would probably end up breaking a lot of the legitimacy (since you'd need over half of miners to agree to it, in which case some sort of "central" entity would exist)

[nwh]

You could to a point, but you would need a lot of mining power and the ability to act quickly.

BIPS didn't announce that they'd been compromised until over a week since the funds were sent out, so it's completely impossible at this point. If you wanted to get a transaction with one confirmation reversed, you would need to convince the two largest pools (ghash.io and btcguild) to mine a fork that doesn't contain your blacklisted transaction in under 10 minutes, and even then they'd create a very noticeable reorganisation. You'd also then have to race to get your funds out, as you know your keys have been compromised.

It's fairly impossible really.