Look, we're stuck with some big businesses; it's hard to have a cell phone without the big three carriers. But there's no reason to use a big bank that doesn't care about customer service. Credit unions have big ATM networks, and most of them have good customer service, and some of them actually have interest rates on deposits (as much as anyone does these days, but 0.5% is better than 0.001%). Even doing banking at a brokerage is a step up vs a big bank.

All that said, a registered letter often helps, and so does copying said letter to their regulator and/or a relevant member of congress. If I had a major issue and couldn't get anything done, I'd try protesting at their corporate HQ.