How do I restrict an FTP user to a particular directory tree?

This is particularly useful if you want to allow them ftp access to only their home directory (so they can control their mail forwarding), or access their home pages but be able to access nothing else.

The best thing to do is to get rid of wuftpd and install proftpd. Proftpd is easily configured, supports multiple anonymous accounts, and permits chrooting of arbitrary users and groups.

The documentation on the proftpd site contains all you need to know about setting up chrooted accounts.