How Close Port 80 on Comcast/SMC D3G-CCR Gateway?

We are experiencing numerous web attacks that all target port 80 on our server.  We do not have any outward-facing functions on the server (no web site, for example).  I would like to block port 80 on the gateway to prevent these attacks from getting through to our server.  In the router/gateway, I have removed the port 80 forwarding from the gateway to the server.  The incursions still occur.  I then lookrf at the "port blocking" function on the gateway (firewall section).  In reading the description of that function, it appears to only affect *outgoing* attempts from internal network workstations to use port 80.  I am concerned with blocking the reverse, namely attempts from the Internet (external) to get to port 80 on our server, inside of the network (i.e., behind the Comcast/SMC gateway).  The normal place to look for a port blocking function is the firewall/gateway/router, but it only seems to work in the "wrong"  (for us) direction.

Another option would be to create a new incoming rule in the Windows Server firewall that would block incoming port 80 activity.  If there is an option, I'd rather do all of the port-related configuration on the gateway, but that may not be possible.

Thanks very much for any assistance you can offer.