|depending on the setup, you should be able to block mac addresses by utilizing the router.|
There is of course the issue of your modem/router already being hacked by said infiltrator. my suggestion is to
1.) go to a restore point before the hack
2.)reset both your modem and your router. (change these both to a secured connection wep/wap encripted)
3.)report the user/hacker to your provider, encluding the mac address and other information.
next thing to do, is to set up your router/modem so that there is a limiting factor on how many connections are permitted on your modem. (this should be set to the number that you utilize) or set it up so that only your mac addresses can connect(should be in wireless setup)