You can delete any client on the Details tab. This will remove them from your client list. It will also:

If your client is connected via the Client App, you have the option to only disconnect them.

This will keep them in your client list but revoke access to your training space on their Client App.

Note that sessions, bookings and direct debits won't be affected and continue as normal. If you wish to delete them as well, you will have to do this separately.