NOTE: If your neighbors have a guest account that you are seeing, there is nothing you can do about them. Then again, it would be their network bandwidth being used 🙂
If your WiFi base station is offering a Guest Network, then chances are it has the exact same BSSID (numeric ID) as your regular network. You can check this by connecting to the Guest network, then Option-Click on the WiFi menu bar icon and you will see details about your WiFi connection, including the BSSID number nn:nn:nn:nn:nn:nn. Now reconnect to your normal WiFi network, and again Option-Click the WiFi menu bar icon to see your normalnetwork's BSSID. If they are the same, then the Guest Network is definitely coming from your WiFi base station.
You could also use the Wireless Diagnostics. Option-Click on the WiFi icon and select Wireless Diagnostics. Then from the Wireless Diagnostics -> Window -> Utilities, select the Wi-Fi Scan tab. Then click Scan and see what WiFi networks can be seen from your Mac including the BSSID values for each (and other fun WiFi network information).
As MrHoffman has said, if this is an Airport Extreme WiFi base station, then you use the Applications -> Utilities -> Airport Utility to disable the Guest network
If you do not have an Airport Extreme, Apple Time Capsule, or Airport Express, then you will need to use the manufactures instructions for enabling/disabling a guest network. Generally this is done via a Web browser, but each vendor has different methods for doing this.
