Can you get to an option screen for your wireless AP? Are you using the default gateway number of 192.168.0.1?
Without knowing the setup you have, I can’t tell ya exactly what you can or can’t do. My router will:
block certain MAC’s
Hard assign IP addresses to certain MAC’s
allow WEP
allow me to change the default router (192.168.0.1) address
allow stealth mode, where the machine trying to connect has to know the MAC address of the router
about four more options I haven’t even tried playing with
Thankfully, today's laptops make that a snap. You can even buy t-shirts that light up when you get close to a wi-fi network. But personally, when I make a connection, I couldn't tell you whether it was peer-to-peer or a client/server setup.
I don't have a home network myself. I'm surprised that you can't tell when someone is using your network, or what they're doing with it. Just another reason I'll probably put off going wireless in this house!