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?

Consider removing the network from your preferred network list (although I know you would like to avoid this) and create a new Keychain item in the Keychain Access App. Use the SSID as the Account Name. This will save and secure your password in the same way that all of your saved passwords are saved. It may add an extra step to joining this secondary network (i.e. retrieving your your credentials from Keychain Access) - but your password is saved and secure.