How to block Internet access for some devices on given hours?

I have a Ubuntu machine (15.10) running as a gateway which routes / firewalls traffic between a LAN an Internet.

I need to block Internet access during some hours for specific devices on the LAN.

Before jumping into scripting iptables rules which will be enabled/disabled via cron, I wanted to check whether there is no such a service/program already existing. Neither a repository check nor Google searches brought in anything relevant.