Prevent files in a folder from being moved?
You can use Access Control Lists for this. Run

chmod +a "USERID:deny:delete_child" DIRECTORY


to prevent the user USERID from deleting anything from the directory. To undo, run

chmod -a "USERID:deny:delete_child" DIRECTORY


PS: If you want to have it applied to current and future subfolders beneath DIRECTORY as well, use

"USERID:deny:delete_child,directory_inherit"