Selecting a default wi-fi connection

You can change the preferred network simply by dragging it to the top of the list.

Lion: System Preferences -> Network -> Wi-Fi -> Advanced

Older OS: System Preferences -> Network -> Airport -> Advanced

You should be able to "forget" the XYZ-Open network in order to never automatically connect to it again.

Wifi connections are connected to based off the strongest remembered one first, if I recall correctly.

Open your Network pane in System Preferences.

Click Advanced there in the bottom right when the Wifi adapter is selected.

Then click the XYZ-Open network and delete it, it should never be automatically joined again.

Since it is an Open Network, there should be no concern in deleting it, as you can always simply rejoin it. What I don't know is how to select it, but doing so without remembering it. That's easy to do when you have to type in a password, but that dialog never appears on an Open network.