To lock a user in Ubuntu sudo usermod –expiredate 1 [username] Substitute the user’s username where username is shown below.

sudo usermod --expiredate 1 username

This also can be shortened to

sudo usermod -e 1 username

To unlock a user

sudo usermod -e -1 username

This will disable the user both from accessing via password and from accessing via a private key.

You can also expire a user at a future date using

sudo usermod -e YYYY-MM-DD username

To expire a password and force a user to enter a new password

sudo passwd -e YYYY-MM-DD username

Related: Managing Users in Ubuntu – Reset Root Password on MySQL Database – Don’t Copy-Paste Directly from Website to Terminal