If you have a look at Security Settings / Firewall Rules and click "Add"
In there you can add a service called "ANY (ALL)" and pick "Block by Schedule, otherwise allow"
Then you can pick a single or range IP address, so its best to have the router to assign the same IP addresses to each computer therefore you know who has what IP address. (You can do this in "LAN IP SETUP" under Address Reservation)
Back at the firewall settings, you can Block by Schedule, otherwise allow the IP address you want. Under "WAN User" leave as "ANY"
Then go to Schedule, in there you can setup a Schedule by time and day for when you want the router to block all traffic.
Make sure you tick "Adjust for Daylight Savings Time" or your time in the router will be out by an hour!
You say you have MAC address filtering on, I hope you also have encryption enabled???
MAC addresses are very easy to fake.