There may be a time when you want a Domain to have a rapid expiration date. If a client is cantankerous, this could be one of those times. There are a few options that you can choose from. One is to transfer it to another registrar so that it is under the control of someone else. You can also change the DNS so that it no longer points to the same site or even any site any longer.
If you want to take this type of an action while remaining professional, tell the client that they need to make an account with another registrar, and you will transfer ownership of the domain to them at that time. This allows the client to have some choice in the matter. You could set a deadline for this to happen, then take your own action if the client does not respond. Your action could be to change the DNS so that it no longer points to any site.
Telling the client what they need to do and explaining that the domain will be changed to point at nothing on the date of your choice if they do not act gets you out of managing the site. The client will have to deal with all associated costs after you complete your action. The client would need to take action by the fixed term of the domain's registration date if they want to keep it. If it is close to the expiration date of the domain, tell the client the deadline. If they do nothing, then you also do not have to do anything. The domain will simply expire. Make sure you cancel any automatic renewals so that you do not get any surprise bills.
If you have already refunded the client and included the domain fee, the domain is yours. You can do anything you want with it, including keeping it or letting it expire. Another option is to contact the registrar directly. If you cannot delete the domain through your management controls, the registrar can. The domain in question will immediately be gone. For more information click here https://www.reddit.com/r/web_design/comments/9ttqyv/setting_a_domain_to_expire_immediately/.