I don't think they can do that for work already done.
I've no idea how the banks work it but it's quite possible these people would still have got their bonuses even if the bank had been allowed to go completely bust. I believe employees are at or very near the top of the pile of creditors and their bonus payments may have come under that.
The "fat cats" who basically write their own remuneration contracts, will just up their salary to compensate. (ETA 1 year to compensate) The middle ranking (good) people will shuffle around jobs to one that pays more salary and less bonus so that salaries will eventually just go up to compensate (ETA 5 years before a new status quo is established) and the people at the bottom will see no difference until there is another boom cycle where they will find their bonuses temporarily capped due to a scheme that was never meant to penalize them. The following bust will then see them with a proportionately lowered income while the "fat cats" will lose a bit of beer money and will keep their inflated salaries from the boom years "necessary in order to recruit the best people".
I don't think the problem is bonuses. I don't even think the problem is large bonuses. I think it's more that, especially for the people right at the top, their bonus seems to be disconnected from any real metric of company performance, either relative to the industry sector or absolute.
If the CEO decides that the company needs to implement XYZ to remain competitive then surely if he is right and XYZ is implemented well then the company will remain competitive and he'll get his bonus. As it moves down the chain though the people who actually implement XYZ need to do a good job at that so their bonus should be tied to how well they do XYZ, even if the CEO was wrong about it being a key project for the company.
Tim.