I don't see any Data ONTAP APIs that do what you seek. Get-NaCifsSession can tell you about users and volumes being accessed, but not files. And I don't see any API to terminate an individual session. You might experiment with manipulating the CIFS share ACLs, but otherwise you would have to call Disable-NaCifs, which is admittedly rather drastic.
You can do it from the Windows' "Computer Management" interface. Open it (compmgmt.msc) and connect to your filer/vfiler (by right-clicking the "Computer Management (Local)" in the first line of the left half of the screen -> connect to another computer). Down there you can find "Shared Folders" -> "Open Files". You can see there all the open files and sessions to your filer/vfiler and by right-clicking the specific open file you want to terminate, a terminate option will appear (don't remember the exact option, but it's there).
I know it is a little late to send you this message but maybe someone else needs it.
I have a script that copy folders from a source UNC path to a destination and closes open sessions of exe files. This scripts if for 7-Mode filers but you can change it for your needs.
<# .SYNOPSIS This is a function to copy system files from a source folder to a target folder.
.DESCRIPTION It is used to copy files from a source folder to a target folder. If any .EXE file is in use it closes the session before copy operation.
.PARAMETER Source Is an UNC source path for system's files (\\filer\share\folder\sub-subfolder\...).
.PARAMETER Target Is an UNC target path for system's files (\\filer\share\folder\sub-subfolder\...).
.EXAMPLE Copy-SystemFiles.ps1 -Source \\filer1\share1\folder\sub-folder\third-level-folder \\filer2\share2\folder Will copy every file in the souce folder to target folder closing every executable .EXE files before copy operation.