There is not a dedicated GUI for FTP only that I am aware of. I think you either need to setup a custom program or something like this -
ISPConfig or CPanel - Disable all services except for FTP (and maybe outgoing mail for alerts). Remove any icons from client portal except for FTP related. Set the default directory to be something like "public_ftp" this way they dont see anything like www or public_html. The the user can go in and create the accounts the way you want them too or you can just create they all from admin.