How can I get my hands on network monitoring tools?

Try the links below, however if this is your router then why don't you just cut him off completely and get your own account and then lock it down such that only you and who you want on the system to come on. Come to think of it you can lock it down further because on the netgear router you can enable MAC filtering. What this does is allow you to selectively allow only those computers on your router that you want on there. Now if he wants to get on then he has to give you his MAC address. With this you then allow only three IP addresses to be given out and now this restricts him even more because he can't hide behind a false IP address. If you see a duplicate address (warning) on the network then you know it is him.