That doesn't work, says "please check the gateway is a valid remote node"
I though about using the router's "Filter Set Configuration" menu, but that only seems to be for ports.
Its a Zyxel Prestige 310 by the way.

Gnart, I'd prefer to do that via standard WinXP stuff or my router. I had some bad experience with FWs in the past.

Considering that I may want to block IPs in the future, I can't make that many static routes (only 2 left). Is there no way of achieving similar results with WinXP ?

In the end, I did what HoweverComma said, using the tip fishturd provided. Should I ever get into such a situation again, I will probably follow Gnart's advice and get a good firewall.

in case you are still listening as fishturd said, an alternate IP.
Note that it doesn't have to be a existing IP just needs to be
a valid one in the eyes of your router.
Send it just use the last set of digits as an unused IP.

I manage networks for various companies and certain sites send people
to a personal web server on the network that displays a warning informing
the person that that page is off-limits, could sent them to yahoo or wherever.

On XP, Win2k, etc. you can add a route to reroute this type of traffic to a non-working IP address...

If you wanted to block access to 61.51.41.21 you would type the following:
     route ADD 61.51.41.21 MASK 255.255.255.255 <nonworking local IP for gateway>

For example, on my network 192.168.1.240 is valid but it is not used so I would enter:
     route ADD 61.51.41.21 MASK 255.255.255.255 192.168.240

This would effectively block access to that IP address. However, if you reboot your system, this goes away. To make the route persistent across boots, add the "-p" switch as follows:
     route -p ADD 61.51.41.21 MASK 255.255.255.255 192.168.240

When you do a "route print" you'll see this at the bottom of the output:
============
Persistent Routes:
  Network Address          Netmask  Gateway Address  Metric
    161.58.201.64  255.255.255.255    192.168.1.240       1
============