There’s an upcoming FR which will be available in the next version of NX – https://www.nomachine.com/FR11P03736. This FR will be enough to cover your case.
Before this FR is implemented you can do it this way:
Set DisablePersistentSession all, in server.cfg, what this will do is that virtual desktops will be terminated upon disconnection.
Next remove everything from AvailableSessionTypes key and leave only unix-xsession-default:
AvailableSessionTypes unix-xsession-default.
This will make default virtual desktop as only available session. After this change, webplayer will start new virtual session upon login. If you’re using nxplayer, nxplayer will show only one icon “new virtual desktop icon” – to start a new session you simply click on it.
To save this behavior the user could switch shown desktops from “all desktops” to “my desktops”; after this change if same connection is used, the user will start a new virtual desktop after login, without the need for clicking a new session again.