Simplest way to block a single IP?

[jan.muenther at nruns.com]

One IP can be null routed as a simple quick workaround:

route add that.ip.add.ress 127.0.0.1

The syntax of the route command will differ from OS to OS, but basically you
set your loopback as the gateway for that host. 

Not a permanent solution of course.

Cheers, J