A client with any accounting history at your business will not be able to be deleted. However, if a client from your client list is deceased, there is an available workaround that would allow you to remove them from your client list.
This workaround would require you to create a New client record in your client list named "Deceased Clients". When creating this client, it is best to enter in a "dummy" email address like "test@sample.com". You would then need to merge the client record of any deceased client into this new client record.
You can find more information on merging a client's record here: Merging Client Records.
This will remove the client's name from your list, and prevent them from receiving any further communications from your business.
Note: This process will cause all previous receipts and transactions in your MassageBook Reports to list "Deceased Clients" as the client's name for all clients merged into the "Deceased Clients" client record.