I don't think the default ftp server that comes with mandrake allows you to prevent that easily. You'd have to make a chroot environment to run the ftp server in which comes with all kinds of problems.
The easiest way in my opinion is to install www.proftpd.org
They have rpm's so it shouldn't be very difficult to get it running. When you've got it installed edit the file /etc/proftpd.conf and add the line:
This will force each user to be jailed inside it's home directory.