Firewall/NAT

I recently have been having some attacks to a SIP server sitting behind a m0n0wall.  The attacker is spoofing the IP address in the SIP registration messages and I am wondering if their is a way to have m0n0wall detect these spoofed packets and automatically reject/block them.

For now, I am just using tcpdump on the SIP server and adding the IP's one by one as block rules.  Which brings up a second question.  Is their a way to have an IP address list for blocking?

The attacks have been unsuccessful thus far and will never be successful since this SIP server is heavily locked down.  It's just annoying.