I have an iBook G4 running Leopard, and I toasted my FireWire controller. This caused all sorts of problems, until it occurred to me to disable the FireWire drivers. You don't need to recompile the kernel: just remove all of the kexts in /System/Library/Extensions that contain the text "FireWire" in their name. Then, remove /System/Library/Extensions.mkext (or, in Leopard, this file will be automatically rebuilt when you remove the extensions).
Reboot, and your FireWire port is no more. Thus, if you are worried about security, and don't use FireWire, you can rest assured that it won't be a vector of attack.
Of course, expecting real security on any machine to which attackers have physical access is a bit foolish.