How to limit the nat's stream speed?

[he ccjj]

Hi,ervery one!

I use freebsd5.4+ipfw+natd to setup a box for sharing internet,it's
work fine.But i have a very serious problem:
Some computer of my inner user was attacked by virus,they make very
big volume of stream to internet,so the natd will occupy almost all
the cpu,the others can't visit internet at all !! Is there a solution
to limit the natd's cpu occupancy or limit every user's stream speed?

Any advice is welcome!