application to block or restrict an IP form contacting

Ok here is the deal I am in need of an app or knowledge on how to block a certain IP from contacting on Yahoo IM or any IM program, someone who uses my wireless preferabbly an app that I can do this on the main computer (without having to install on the PC I need it to block it from). I have checked my router config. and cannot find a way to do it. Does anyone know of anything that can do this?
Thanks
Answer #1
Think some routers support seeing other users , some dont.
Hmm you could try filling in the number of your router in an internet browser, (or was it an other number) ?
I`m not sure, but I have done that some time ago, and in that page you could accept ports and deny, of which I had to open ports to be able to play World Of Warcraft..
So try filling in your routers number or so, or IP? Its been a long time ago so I cant really remember what number to fill in. This method should not require any software btw.