Actually you could do that. Take a backup first, just in case anything goes wrong. Mark the server as decommissioned. Delete the websites from your panel - they won't be deleted from the server since it's now disconnected from your cluster. Then delete the server from your panel.
They will still be using your Enhance licence and their sites will continue to back up to your backup server.
Of course they will have no way to manage the websites. It would be better to give them a fresh server with a full control panel installation of Enhance. Then transfer the websites using the "download website data" option on the website dashboard and import to the new single-server cluster.
