To enable MAC address filtering in FreeBSD first you need get MAC addresses for all clients which you want to give internet access.
To get MACs I suggest use FastResolver. You need to run it any computer located in your network. Also You can run
# arp -a
in your router.
After that edit ethers file
$ vi /etc/ethers
and add ip and MAC address list like:
Use 00:00:00:00:00:00 for that IP addresses which should not have access to internet.
Next step is to delete the current configuration of ARP entries:
$ arp -ad
And finally load the configuration file:
$ arp -f /etc/ethers