Thanks to ASUS tech support for this tip.
Assume you want to prevent all computers on your LAN from sending packets out from their port #12345. Do it this way.
Firewall tab ==> General ==> Enable firewall ==> Yes
Firewall tab ==> Network Services Filter ==> Enable LAN to WAN filter ==> Yes and Filter table type ==> Blacklist
LAN to WAN filter table:
Leave the first 3 columns blank (this is the non-intuitive key)
Enter 12345 in column 4
Set Protocol column as desired
Assume you want to prevent all computers on your LAN from sending packets out from their port #12345. Do it this way.
Firewall tab ==> General ==> Enable firewall ==> Yes
Firewall tab ==> Network Services Filter ==> Enable LAN to WAN filter ==> Yes and Filter table type ==> Blacklist
LAN to WAN filter table:
Leave the first 3 columns blank (this is the non-intuitive key)
Enter 12345 in column 4
Set Protocol column as desired