When you are editing a file, you can lock the file so that other users cannot overwrite it. If you use WebDAV Drive Mapping or DriveHQ FileManager, file locking usually happens automatically. If you want to manually lock / unlock a file, you can use DriveHQ FileManager or any web browser.
In your case, it seems you need to share the file with read-only permission. This way, your sub-users cannot overwrite the file. However, you cannot really share a single file; otherwise, if the file is overwritten, moved or renamed, the share will become invalid. The share permission can only be applied to a folder.
You can create two folders, then right click on one folder and share it with read-only permission and share the other one with full access permission. If you don't want your sub-users to overwrite a file, just put the file in the read-only folder. If you use our Group Account service, by default, the system creates two folders: GroupRead and GroupWrite. The GroupRead folder is shared with read-only permission and GroupWrite shared with full access permission.
You can share the same folder to multiple users with different permissions. You just need to share the folder multiple times using different share names.
Lastly, if a file is locked, then it cannot be deleted or overwritten. I just tested and it works fine now. (The implementation may not be consistent if you rename a file or move the file to another folder). However, in your case, it may not be a good solution as a file lock will expire after certain amount of time.