Shadow file help
 

As a part of linux hardening

In shadow file all Application accounts which are not locked must contain only an asterisk “*” in the Passwd field.

But how would i do it by using command?

Is there any way other than modifying shadow file to accomplish this task?

You can do this with chpasswd. This command reads username/password pairs from stdin and updates the password of the user.
If you supply the -e option, then the password is considered to be in encrypted form. Hence, a command like this:
will change the password field for the user: user in /etc/shadow to *.

This command will also change the "date of last password change" field in /etc/shadow.