If you create a client by mistake (or just made it for testing) or want to remove it for some other reason, you can delete it completely. Tread carefully, though. Usually, it is better to inactivate a client (that way, you can always get the client back should you need to).
To delete your client, follow these steps:
Go to Clients
At the end of each client row, when you press on the three dots (context menu), you can choose the option Delete.
After you confirm that you want to delete your client, all data will be removed.