The Supreme Court has upheld binding arbitration agreements in the last few years, and unfortunately "demonstrating incompetence" isn't grounds for voiding this. There's also nothing here to indicate that customers or the bank were not "competent". There are certainly issues with the fraudulent accounts, but the entire relationship with the bank would likely be covered by the arbitration clause from the initial account setup (which the customer agreed to). Customers would have to show something along the lines of fraud in the inducement.