How can I restrict users from being able to exit the Net Control 2 program by clicking "Exit" command of Net Control 2 Server menu (User module)?

You need to set up a Net Control 2 Administrator password on User computers:

a) remotely, from Administrator computer:

run Net Control 2 on Administrator computer;
select User computers;
click menu command [N]-> Options -> Remote Computer Options;
on "Security" tab, in "Administrator password" field, specify any password and click OK.

b) you can also change these settings on each user computer:

click icon in the System Tray;
choose "Configure" menu command;
on "Security" tab, in "Administrator password" field, specify any password and click OK.
new profile will be applied after restart of Net Control 2 service.