The default library is /users/yourname/music/itunes. Just go to that folder and press Command+I. Then you can do either one of the following. You could do both but you don't have to.
- At the top of the File Info dialog box, in the General section, check the box labeled "Locked." A lock will appear on the folder.
- At the bottom of the File Info dialog box under Sharing and Permissions, make all three settings "read only."
After you do that, iTunes can no longer create a library in the default location. If it tries to revert, it will prompt you for the location of the library. That's annoying, but it is much, much better. You don't end up with media files scattered.
If you have files in both places, copy the files from the internal library into the external library into the corresponding folders, then Open them with iTunes to add them to the library.
This is one of those things, like duplicate files, that is not a bug, annoys the crap out of people, but doesn't cause customers to go away. So Apple will never fix it.