How to force Mac OS X ML not to auto connect to Wi-Fi
Go to the network preferences, select "Wi-Fi", and click "Advanced..." (you may have to click the lock in the bottom left first).  Then, in the "Preferred Networks" list, delete any that you don't want. 

The passwords are saved separately, in the keychain.  I have verified that removing an SSID from the list in the network preferences doesn't remove it from the keychain, so you can delete them without worrying. (this is unlike the case with iOS, where forgetting a network also forgets the password)

You may also want to uncheck "Remember networks this computer has joined" to prevent them from being re-added when you join them again.   Again, this is separate from the "Save password in keychain" that saves the password in the keychain when you enter it.