To lock, you can use the follow command:
# passwd -l username (where username is the login id).This option is used to lock the specified account and it is available to root only. The locking is performed by rendering the encrypted password into an invalid string (by prefixing the encrypted string with an !).
After that, if someone try to loginusing this account, the system will return:
# su - username
This account is currently not available.
To Unlock the same account
Following command re-enables an account by changing the password back to its previous value i.e. to value before using -l option.
# passwd -u usernameThis removes the '!' in front of the encrypted password