The supported types of sessions are defined in the AvailableSessionTypes key in the server configuration file. This key is populated automatically during the installation of NoMachine according to resources present on the system and eventual limitations of the license. See the Appendix section of this article for a complete list of session types and their description.

In order to disable a certain type of session, it's enough to remove it from the list of types in the AvailableSessionTypes key in the server.cfg file. This will apply to the whole system, also in case of a multi-node environment (Enterprise Terminal Server + Terminal Server Nodes).

In multi-node environments, edit the AvailableSessionTypes key in the server configuration and in the node.cfg file on each of the nodes where you want to prevent this kind of session types. Then, on the main server host synchronize information by executing the 'nxserver --nodeupdate NODE:PORT' where NODE:PORT is the name of the node as it appears in the output of the 'nxserver --nodelist' command.

Session types available on the node can be a subset of session types available on the main server.
 

How to disable custom sessions

For example, to remove the option of creating custom sessions (i.e. running single applications) and make the user to get directly the virtual desktop:

step 1: edit the server configuration file, /usr/NX/etc/server.cfg and make a copy of the AvailableSessionTypes key in order to preserve the original settings.

Comment out one of the two keys (prepend the '#' char to the key name).

step 2: remove all the types related to custom sessions from the other key:

unix-application
unix-default
unix-console
unix-desktop
unix-remote
unix-remote-custom

This will affect both connections via NoMachine client and via browser.

On our machine, the final result was:

#AvailableSessionTypes unix-remote,unix-console,unix-default,unix-application,physical-desktop,shadow,unix-xsession-default,unix-gnome,unix-xdm

AvailableSessionTypes physical-desktop,shadow,unix-xsession-default,unix-gnome,unix-xdm

step 3: restart the NoMachine server and all services:

/etc/NX/nxserver --restart

Note that this will terminate all the sessions running on the server host.
In case of a multi-node environment (Enterprise Terminal Server + Terminal Server Nodes), restarting the main server doesn't terminate sessions running on the remote node(s) but disconnect them.

How to disable RDP virtual desktops and/or XDM virtual desktops

In a similar way, it's possible to disable the possibility of running RDP virtual desktops by removing the windows type or disable creating XDM virtual desktops by removing the unix-xdm type.

For example:

#AvailableSessionTypes unix-remote,unix-console,unix-default,unix-application,physical-desktop,shadow,unix-xsession-default,unix-gnome,unix-xdm

AvailableSessionTypes unix-remote,unix-console,unix-default,unix-application,physical-desktop,shadow,unix-xsession-default,unix-gnome,unix-xdm
 

Appendix:

In order to retrieve the list of session types available on the server, run the following command:

/etc/NX/nxserver --resourcelist --class session

Session types and their descriptions are:

(*) Connections to the physical desktop are supported only for specific users.