The Disable this account option is unavailable.

If you are unable to deactivate the guest account, check the SMB settings. Make sure the Enable Local Master Browser option is deactivated at the following locations:

• For DSM 7.0 and above: Control Panel > File Services > SMB > Advanced Settings > Others.
• For DSM 6.2 or below: Control Panel > File Services > SMB/AFP/NFS > SMB > Advanced Settings.

If you are still unable to deactivate the guest account after making the above changes, please contact Synology Technical Support.

If you encounter issues finding your Synology NAS via Windows network discovery after deactivating the Local Master Browser, please refer to this article for more information.