You can restrict the login access of a particular user using two ways

1)Using the passwd command

To lock the user from accessing the desktop environments like gnome desktop ,use the following command.

passwd -l username

Ex: passwd -l user1

This command will block the gnome desktop access for the particular user but you can access the restricted user through the terminal.

2)By editing the /etc/passwd file

Open the /etc/passwd file using the root For restricting the user login,give the user information line and edit the shell type Change the shell type from /bin/bash to /bin/nologin.

Now save the file and exit.

Now the users are restricted from accessing the desktop environments like gnome desktop.