For some reasons, they don’t provide a simple GUI switch for this and their wiki page doesn’t have the correct commands.
Anyway, the way to do it is this:
Check what is your wifi interface. Go to status/Wireless and check the Interface field below (you need to connect at least one client). In the following example, we assume it’s eth1.
Go to Administration/Commands
Type the following (assuming your LAN is on the 192.168.1.x range):
insmod ebtables insmod ebtable_filter insmod ebt_pkttype insmod ebt_ip ebtables -A FORWARD -o eth1 -p ipv4 --pkttype-type multicast --ip-source ! 192.168.1.0/255.255.255.0 -j DROP
Press Save Firewall and reboot.
Voilà! No more multicast flooding of the wireless interface! This is handy if you’re using IPTV. As a bonus, normal multicasting on your LAN will still work (Samba, Homegroups, etc…)