Ya, turn the WEP on. I'm not sure that Airport support 256, but you could try. If it doesn't work, drop it down to 128.
Another thing you can do is enable MAC address filtering which will allow only certain cards with specified MAC addresses (a kind of serial number) to connect. You then add your Airport card's MAC address to the list and none others.
And finally you can disable SSID broadcasting and change your networks name. This feature is available on some of these routers, but not all. The SSID is your network's name. It is usuallly broadcast to help people find it and connect to it. If you disable broadcasting, a user would need to know the exact name to connect. Obviously with this approach, you'd want to change the SSID to something non-standard, so it can't be guessed.
This is about the most you can do easily and practically. It will keep most people out.