System Preferences > Network > AirPort > Advanced > AirPort > Preferred Networks (click the "-" to remove those networks from the list, if they appear there)
Also, make sure "Ask to join new networks" is checked on the Network pane.
Open System Preferences->Network tab, Advanced button, TCP/IP and make sure only your wireless network is there. Then go back to the main Network pane and click on the Lock button to lock the System Preference pane.