How do I prevent OS X 10.8 from autoconnecting to a specific wireless network?
From another answer: How to disable automatic wifi connection?


  The only way to stop from automatically joining a preferred or known wifi network, is to not store the password for that network and type it in each time without clicking the "save password" button.


If you have a network remembered, and the credentials stored, there does not appear to be a way to prevent the system from trying to connect to it on sight.

Another answer with a somewhat related conclusion that may shed more light on the way the wifi autoconnect works: Prevent my Mac from connecting to an unsecure Wi-Fi network?