I will try and keep a long story short but we have been trying to get a terrible client on the straight and narrow for too many years. After he sent an abusive email last year we dis-engaged him but as he had been billed for accounts work in advance, agreed to complete the contracted work. 13 months later we still haven't managed to finally sever ties. We still pay his software subscriptions for which he owes us money. Every time he emails it completely ruins mine (or whichever colleague's) day). I will be sending him a final letter to tell him that's it - we will only communicate with your new accountant in future - but wondered how ethical it is to block receipt of any future emails from him so we can get on with our lives and avoid the stress.