How do you cancel an automatic connection to a once joined wi-fi network on 10.7.4?
Go to System Preferences > Network.
Select your Airport/WiFi connection on the left.
Click on the "Advanced..." button in the lower right of panel.
You should see a list of "Preferred Networks." Select the one you want to "forget" and click the minus "–" button below.
You can also un-check "Remember networks this computer has joined" – that will cause the computer to – you guessed it – NOT remember networks it has joined! :-)