Who decides what's classed as rude, abusive or discriminatory?
As @RedToothBrush says, there needs to be an independent appeal panel, which reviews decisions because, at the moment, there is no real scrutiny.
I work in a public-facing job (GP). The level of abuse we get from some of the public is astounding - including bringing weapons into Reception, the worst racist abuse imaginable etc. We can remove patients from our list but only after warnings, unless they do something violent enough to involve the police coming to surgery. So most people at risk of being removed have the opportunity to change their behaviour.
Anyone removed can just register at another surgery, unless they have been violent, in which case they still have access to a GP but through a special practice. Banking needs similar safeguards. I would defend banks' right to protect their staff from violence and racist abuse etc, but there needs to be some sort of option, even for people who have been violent. An act of violence cannot disqualify you from being able to participate in normal life, beyond any prison sentence you receive.
Discriminatory is nonsense because it is perfectly legal to think that men are better than women, Pakistanis are better than Indians, straight people are better than gay, or whatever. I would disagree with all those views, but people are free to hold them. Unless they lead clients to be significantly abusive to a member of staff, clients' views should be irrelevant to their bank.