(unfavourable comment about SG and grc.com deleted to save space)
I think you may have misunderstood what you read there (or I have misunderstood what you're saying). He says that some hardware routers might be actively rejecting port 113 packets in order to let the requesting server know that you really are there, you're just not running an ident server. This means your IP address doesn't look like a black hole - if someone probes port 113, they will see that there is a machine there, because it's acknowledging their packet. His suggestion is to tell the harware device to send the packet to an invalid IP, so it becomes a black hole again (nothing will ever send back a rejection or confirmation). You could certainly turn port 113 into a black hole with linux, or have it reject the packets (which may solve your IRC problem anyway - hardly anyone required valid ident responses).
He also mentions that ZoneAlarm does a little extra checking to see if the ident request is coming from someone you tried to connect to, and responds if that's the case, or drops the packet otherwise. If this is what you want to achieve with Linux, then I'm not sure how you'd go about it