I was interested in this also, but there is no simple clean way to do this that I have found. I ended up just looking out the window and finding some guy parked outside with a laptop. Knocked on the window, short exchange, pulled the plug on the modem for 5min. and he took off. Or you could just endcrypt your network, but I haven't been able to get WPA to work on a 1Ghz TiBook, so I have to shut it off.
BTW, FIRST POST ON MY NEW POWERBOOK! ITS THE 15" AND I'M GLAD I DIDN'T WAIT.
First thing to try is disable SSID broadcasts, use WEP encryption.
Second thing is set all your DHCP to manual DHCP and type in physical addresses for all your computers.
Third, if your router supports QoS, assign bandwidth priorities to your local IP address. So if someone tries to flood the bandwidth, the router will prevent those IP addresses from taking all your bandwith.
While WEP encryption will stop very casual users, it won't stop anyone who really wants to use it since there are a variety of freeware tools out there which can crack WEP. WPA is better. But WEP is better than nothing.
If you're not confident with the manual DHCP route or have trouble making it work, you may want to consider a MAC access list which can generally be added through the browser interface of the router. While MAC addresses can be spoofed, in combination with other security methods, it's probably enough on a home network.
Check on your wifi router itself for some form of client management software built-in which allow you to see what computers are on your network. If you're using the Airports, then you can download client management software from the Apple support site.
Most WiFi base stations have some sort of indicator lights indicating transmit/receive activity. I would first check that with your own computer(s) off/asleep when you experience intermittent connectivity. If someone is leeching bandwidth, it should be obvious. However, the more likely culprit in my opinion is that you're sharing/overlapping the same channel with some other network in your area. That can lead to intermittent access. 802.11b uses 11 basic channels. Many base stations default to the same channel (channel 1), and people rarely change the default setting. Try changing yours to a different one.
In addition to WPA or WPA2 encryption, create closed network (aka not broadcast SSID) and use Access Control (aka enable MAC filtering), you can also download Airport Management Tools from here and use AirPort Management Utility.app to see if any strangers (or maybe familiar intruders?) are connected.
If you get into your router's setup it normally shows all computers connected to your network. When I noticed an iBook was connected to mine I knew there was a problem since I have a PowerBook, a Power Mac and a dell laptop...so I turned on the WEP and haven't had an issue since. My internet is just labeled "Brunswick" which is the name of our street so it might have been someone thinking there is a local connection. Who knows? They aren't on it now lol.