In order to protect public data from others, simply add sticky bit
on the directory, so only root, owner of the directory or owner of the file will be able to delete the content of the directory. To set it:
~$ chmod a+t /path/to/directory
You can also consider adding the SGID permission on each directory according to the group it belongs, so only those users will be able to alter/modify the data inside it, who are member of the group.
~$ chmod g+xs /path/to/directory
In order to set prompt, simply append following line in every user's ~/.bashrc or ~/.profile file: