Dynamic firewall based on bandwidth usage ?

Hello,
I have a common problem but cannot find a solution.

My setup :
all servers are Redhat Enterprise 4
CISCO PIX in front on a HTTP load Balancer/failover  (called a director 
in the L.V.S. jargon) that sends requests to 4 web servers (cluster 
setup  based on Linux Virtual Server include in redhat cluster suite).

Now my prob :-)

From time to time users download our site and block all http connexion, 
and worst, use all our bandwidth. So I have to block (or redirect) those 
network abusers after a download limit (for ex : 1Gb per day) for lets 
say 1day.

Because of the director, I cannot use the apache2 mod_cband.

My first though is to look at the iptables on the director but I cannot 
find any information about that kind of setup.

Do you know if it is possible using build in linux tools(iptables ?).

If not, do you know some hardware appliance that could do that ?

Thanks !