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

As soon as you save the password the network will be added to the preferred list even if the remember networks button isn't selected, and it will connect to it automatically (or rather it will connect to the first one on the list that is finds in the order in which you have them listed).