(Oh hey, a riddle for me to flail at uselessly until I render it unsolvable. Let's see...A bank is a legal structure set up for making loans and taking people's money with which to make loans [giving them part of the interest on the loans]. There are many ways to hurt a bank, but most of them are rather destructive and cost a great deal in various ways: there's taking out a loan [or several with other people's IDs] then spending all that money on avoiding the repercussions of defaulting on the loan[s]; there's tricking the bank into making bad loans to other people who will default; there's destroying enough bank records that other banks can leverage them out of business, or so much the bank can no longer keep track of itself; there's convincing people to withdraw their money from the bank or not use it in the first place; and there is, by the bankers' definition of hurt, simply never letting them use your money, and depositing it in a credit union.
We could also be talking about the older, more physical banks, at which point you're reduced to simpler forms of thievery and cheating.
Edit: It occurs to me now that the question might be talking about "bank" in a more figurative sense, such as "amount of currency I have available", or a case of where we have some coin being completely useless, and we could turn it into something useful. If this refers to Oric's potions of giving us magic for a boss fight or providing us with loot for our exploration, then the probable response is most of us actually want to see Ciro get five BLOD before L5.)