At my old company we would ban customers that were repeatedly abusive to customer service agents. Agents had the right to hang up on customers that were being abusive and if the same customer kept getting reported, eventually they would receive a letter from the legal department telling them to stop. If it continued, they would get banned.

I remember one guy was so bad that a director got the phone system to automatically route any calls from him to his mobile line and put him in his phone book. He would very politely greet him by name as soon as he picked up the call to make it clear that he wasn’t ever going to get through to anyone else.