Linux block access to router login page

I am running virtual machines with Gnome desktops on Debian Squeeze and Wheezy. The virtual machines are on a different subnet than the host. I would like to block the virtual machines from accessing the router/firewall login page via Firefox, but still have access to the Internet.

Is there some way to do this with ufw, iptables, host.deny or possibly on the router itself using an address range (I can use Blocksite and lock down the config, but would prefer some other approach)? I've tried various things, but either my attempts fail or I block access to Internet from the VMs.

Thanks!