Well...The firewall is doing its job. It is hard to track someone doing that unless you have other "traps" setup at other points, for instance your router.
Sometimes I have noticed that on my network and it ended up being someone within the company trying to do weird things and it looked on the firewall that they were doing spoofing when actually they were just doing something incorrectly.
Does that help? Probably not, but at least you can rest assured that the firewall is protecting your network.
