To disconnect a Client accounting file, delete them from the Client Connections and Client Settings pages in the Clients board by clicking the three dots icon under the Links column for a Client file.
*This will delete the client file and all its data permanently.*
You can also delete your Client file by clicking the Bin icon to the right of a Client file on the Client Plans page.
To add a Client for workflow only, you must re-add them as a Workflow-only Client and add them back to your Automation schedules.
Please read: How To: Add Workflow-only Clients
If you are looking for information to share with your client, you can find some handy resources here.