Deactivate/Activate User - Shell Command

Hi all,

I am looking for a shell command to deactivate and activate a user. We want to prevent some users during specific timeframes during the day so they can't login with their VPN clients. We are already using a shell command / cronjob to deactivate/activate Site-To-Site VPN tunnels. Does anyone know such a command for users? Couldn't find anything yet online.

Thanks.