# disallow peter from logging in
sudo usermod --expiredate 1 peter

This is how you can reenable that account:

# set expiration date of peter to Never
sudo usermod --expiredate "" peter
 1930  sudo usermod -L -e 1 hatia
 1931  sudo usermod -L -e 1 duret
 1932  sudo usermod -L -e 1 faure
 1933  sudo usermod -L -e 1 guidotti
----
sudo usermod -L -e 1 [username]

and this to unlock an account so locked:

sudo usermod -U -e "" [username]