Prevent Folder/Directory Creation by User or FTP

I have spent much time trying to figure out how to prevent folders from being created in a specific directory.

I'm running FreeBSD with, I believe, a typical ftpd installation. I'm not overly familiar with installing or configuring programs on Unix, but I can navigate around and edit config files.

What I would like to do is prevent additional folders from being created in a specific directory which is used by the FTP server. I don't care if I have to do it on a FTP server level, or on a Unix system level. I do however still want to allow files to be written. The hope is that we will prevent users from uploading via FTP a tree of folders with small files in it. I am trying to prompt them into sending only compressed files such as ZIP.

I've looked at chmod, chflags, etc. but none seem to provide that kind of functionality. Isn't there a way to specify that files can be written but not folders? Alternatively I wouldn't mind disallowing the "MKD" command for a specific login to our FTP server. Reading through ftpd documentation has not really been helpful to me; perhaps I am missing something.

Thank you for anyone who is familiar with this problem and can help provide a solution.