How to disable a user's login without disabling the account under Linux-Unix

Let's say I create a user named "larry" using the adduser command. How can I make sure this user will not be able to login into the system directly, without disabling the account. In short, I want the account to be accessible via su - larry, but I do not want it to be accessible via a regular login prompt.
 

There are two methods to prevent a user from being able to login:

1. you can lock the user by editing /etc/passwd
2. by directly issuing the passwd command with the -l switch

Method #1

1. open a terminal and login as root
2. type vi /etc/passwd

Now you are in passwd file press i to edit the file.

Change the below line with the nologin option (/bin/bash means the user is able to login).

root:x:0:0:root:/root:/bin/bash

to this. nologin means the user is unable to login.

root:x:0:0:root:/root:/bin/nologin

Method #2

To lock user: passwd -l username
To unlock user: passwd -u username