Xcode 5: disable single-click in navigation
There's no option to disable the single-click behavior. Two options that get you close to your desired behavior are:
Use a single separate window for most of your tabs. Use a "main" window that has the file navigator visible, and a separate "work" window with the file navigator hidden. If you want to add a tab to the work window, create it in the main window and then drag it's tab over to the work window. This is an extra step, but you'll never have a single click change any of the tabs that you care about (thought it'll still change the primary or focussed editor in the main window, depending on your settings).
Use separate windows for each file. There's a preference setting that lets you create windows instead of tabs when you double-click a file in the file navigator. Use Mission Control instead of the tab bar to navigate between your files.