It's impossible to undo because a lot of bids are build on the top of other bids. You would need to undo tons of history. A lot of legit players and traders would lose money. They would sue for market manipulation.

Let one who makes mistakes to pay for them.

However this does not prevent doing other trades on the top this trade to other direction. It is the question who catches the falling dagger and pays for the mistake of others.

See a similar story here

https://en.wikipedia.org/wiki/Knight_Capital_Group

Responding to peer comments:

This is pretty clearly NordPool's fuckup. There's no way in hell this bid should have gone thru without human review. Seems clear that their UI is to some degree - in some critical spot - fragile, user-hostile, ill-designed.

You can say that in analogous cases, finance firms have been put out of business by bad bids caused by slipped fingers. Granted. But in this case you're throwing a wrench into a power grid, with the potential to cause a system disruption, in winter no less.

There are many market participants, some of whom start trading on this information immediately. The Finnish grid is connected to the rest of Europe so a market participant could in principle flow this cheap energy from Finland all the way to Spain if the cables had capacity. I would guess it was too late the moment it happened.