How to block a device from internet with OpenWrt

You could use LuCI Access Control, but it doesn’t work with Adblock, Adblock will fail to start if you use LuCI Access Control.
Go to Network, Firewall.
Click Traffic Rules.
Under “New forward rule:” enter a name for it, and use the default source zone and default destination zone.
After clicking Add and edit, select the mac address under “Source MAC address”. Change “Action” to “reject”.
Click Save & Apply.
Now either reboot the router, or SSH into it and type “/etc/init.d/firewall restart”.