Technically, it's the shareholders that pay the fine although, of course, like any business the bank can then recoup its costs through higher charges to its customers, but those customers are free to move their business.
On the bankers themselves, IANAL, but as I understand it, there is no law against incompetence or deliberately making a complete hash of everything in a privately-owned company. (I think there may be something on maladministration in a public office.) Where these has been genuine fraud, there may be a chance of prosecution, but I don't think you can (or should be able) to throw someone in jail simply for being spectacularly bad at their job. However, in most significant financial roles, the regulator can determine that you are not fit and proper, which makes you pretty much unemployable, except by Wonga.