gnome gconf remembers networks you have been connected to and stores them in a preferred networks list, then at the login it choses to automatically connect to the network with most powerful signal and less encryption.
“Preferred networks list” is located at .gconf/system/networking/wireless/networks/ folder in your home directory (note the initial dot, it is a hidden folder!).

Inside networks/ folder you will definetly find a few folders each one with a network name you have been connected to. Simply delete those you do not want to automatically connect to any more. (ie. the one of your neighbour)

Have a nice cup of Ubuntu!