I'm with Jean-Michel. I'm the only teacher using my question bank. I'm also the site administrator and recently told the other chemistry teacher at my school that if she wants my questions then she'll need to import them into her question bank rather than sharing them. Quite frankly I don't trust anyone with editing rights on my questions.
Which brings me to my point: If you're working with shared files with shared editing rights then you should expect shared editing. If you don't want shared editing then why are you using shared files?
I think that if you want to share questions then they should have a defined owner (and possibly a group) who has editing permissions. Here are a few scenarios:
1) Teacher A owns a question that has been shared with Teacher B. Teacher A wants to edit it and propogate the changes down to his quizzes. Teacher A edits the question and all the quizzes are updated.
2) Teacher B is using the shared question owned by Teacher A. Teacher B is using a shared file and should expect that Teacher A may update it at some point.
3) Teacher B wants to use a shared question owned by Teacher A but does not want to worry about Teacher A making changes. Teacher B then makes a copy of the shared question to use in the quiz. Since Teacher B is now the owner of the copy, he can edit the copy and only affect his students.
4) Teacher A and Teacher B work collaboratively on the questions and decide to give each other editing rights to all the questions that they own by using a group. This is done so that any mistakes or improvements can be made quickly by whoever finds the mistake and propogated out.
I'm sure there are many options I'm missing in here but that is some thoughts to start with.
To me what is natural and expected is the same way that many computer systems handle shared files. Define an owner who has read, write, and delete permissions. Define a group that can have any combination of read, write, and delete permissions. Allow the owner of the files to assign permissions as desired.