MacOS – Stop sharing Macintosh HD and home folder with File Sharing

If you set up their accounts in the Accounts pref pane on the server as "sharing only" then they will only get access to shared folders set up in the Sharing pane of System Preferences.

See the Apple docs on creating a sharing only account.

Since you say the email backup software requires that the user has real folders, then you can't remove them as "shares", they have full access to these folders and the server will advertise that.

Without knowing why the email software requires the user to have folders then a workaround is difficult to find.

According to this answer on the Apple Support forum, if you enable (tick) a user for SMB sharing, it shares the root of the system drive (provided they are an Admin):

Users you have set up with accounts in Accounts preferences can access your computer by connecting to it over the network. A user with an administrator account can access your entire computer.

You can see the drive because you are an administrator, and you entered your password on the windows machine. If you hadn't entered your password (or had used a non-administrator account) you wouldn't have been able to see the drive.