this should be added to trac as a feature request. There are a couple of places it could be changed:
1) From the orbiter, you can delete a file. This should only work if the user is checked as "Allow to modify installation"
2) From the web admin, you can delete a file -however, the last time I checked, it will not allow you to do so if "Allow to modify installation" is not checked.
3) Directly from a command line, over ssh or as a Samba share I don't think it would be feasible. For users that should not be able to delete files, you will have to control what they can access and how they can access it. Using the Orbiter and/or the web admin would be the recommended ways.
Also, remember you have your own private folder to keep your files safe - you may need to incorporate that into your plan as well.