FileSharing Assisant

Apple removed the FileSharing GUI from macOS Server 5.4. :-(

Precursor FileSharing Assistant is a free FileMaker Runtime utility for High Sierra that can list connected FileSharing users and disconnect selected users.

Download FileSharing Assistant v1.0

Documentation
F.A.Q.

Requires macOS Server.app (because it utilizes command line arguments only available when macOS Server is installed).