No, it’s unacceptable for a customer service agent to hang up on a rude customer and they must never do so. An agent should try to talk to a customer even if they are insulting and try to get to the bottom of the issue. The agent should aim to clarify the situation with the customer and calm them down. If they can’t reach any conclusion or agreement, the best thing to do is to interrupt the customer, inform them that the conversation will be continued during another call, and propose a date for that to take place.
Additionally, to prevent customer service agents from hanging up on customers, make sure all your agents know that calls with customers are recorded. Not to make them feel scared or controlled, but because they should know that their managers can always go back to the recordings, check how the conversation went, and support their team members in dealing with demanding clients.