Right! That confirms what I thought. You have the "Advanced Proxy" addon installed, which is where you saw the "Limit per host on Green" setting. This setting and the others on the "Advanced Proxy" page are only applicable if your clients are being forced to use the proxy. The easiest way to control web traffic (at least on port 80) would be to check the "Enabled on Green" and "Transparent on Green" options.
This will have the effect of redirecting all port 80 traffic that tries to traverse your firewall. Thus you will be able to apply your speed limiting settings. It will not however trap any traffic on other ports. The advantage of this is that you will not need to make any changes to the client machines, hence the name "transparent proxying".
If you do want to control all ports, you will need to block all outgoing client access at the IP level using iptables, and then notify all your clients to enter the details of your IPCOP in their proxy settings. If not, all non-port 80 traffic will be blocked.
If you want to do the above, I can supply the appropriate iptables commands.
