In order to do this, you must
- Be using ProFTPd.
- Have a virtual FTP host setup for the site, or it should be on its own IP.
Open up the file /etc/proftpd.conf for editing.
Put these lines in your ProFTPd config file, under the relevant virtual host settings.
<Limit DELE MKD RMD XMKD XRMD>
This will deny all users with the exception of "usrname" the ability to make, or remote directories, or to delete files in the directory "/path/to/folder/".
Hope this helps! Any further questions and I'll answer them in this thread, please do not PM me.