you can't do nothing in there for blocking. your going to have to eather just unplug her computer or change it to a IP that will not work on your network like 192.168.55.1
what kind of router do you have? and if there really is no options you could just give her a static IP that will not work on your network so she cant get on the internet.
the farther you are away the less bandwidth you will get.
falco make sure you have the newest drivers for your network card, old drivers can cause the problem.
both of these are very good, the first one i have personal expereince with and the card is great
http://www.newegg.com/Product/Product.asp?Item=N82E16815116628
http://www.newegg.com/Product/Product.asp?Item=N82E16815100134