I need to prevent standard users from creating new user accounts in ubuntu-12.04-desktop
As Eric Carvalho says, only administrators can create new users. Standard users cannot do that.
When an administrator creates a new user, it can be an administrator or a standard user. But no standard user can ever create a new user account.
The user created when you install Ubuntu is an administrator, not a standard user. You should always have at least one administrator account--if you were to make all the users standard users, then you would be unable to perform any administrative tasks (though this can be fixed).
Perhaps you have an administrator who you want to be a standard user? If so, you can change that in any of the same places where you'd create new user accounts. If that's not working, make sure the user who you want to be a standard user is not a member of the
If that doesn't work or you need help doing that, then please edit your question to provide as much additional information as possible, including the output of