What I would suggest is to set up the seperate FTP account Then you can either:

1) Use file manager in cPanel and set the permissions of the folder (right click on it and chose 'Change Permissions') so there are no 'write' options enabled.
2) Use an ftp client to login to the server with your main account and change the permissions to that folder.

Hope that's useful, particularly if you want to do it all through an internet browser