If you are looking for ways to lock-down your ProFTPD server without using iptables this may help. I recently had to find a way of securing a VPS without iptables. The ftp server only needed to be accessed by a few known users so I ended up with two reasonable possibilities.
I opted for the last choice. To configure this you need to modify the /etc/proftpd.conf file by adding a
<Global>
AllowOverwrite yes
<Limit ALL SITE_CHMOD>
AllowAll
</Limit><LIMIT LOGIN>
Order Allow,Deny
Allow from xxx.xxx.xxx.xxx (IP's to allow)
Deny from all
</LIMIT></Global>
Hope this helps someone!