[Dovecot] Need a quick, safe method to empty /home/user/Maildir/{.Junk, .Trash}

I have some (Thunderbird client, dovecot-1.0.13, Maildir) users who get an appalling amount of spam-with-attachment, and it's causing backups to take an inordinate amount of time.

I'll implement some quota and server-side spam management when I go to dovecot-1.1, but in the meantime:

What is the safest way to empty all messages within, but not delete, the following folders from the server command line:

/home/user/Maildir/.Junk /home/user/Maildir/.Trash

I don't want the Thunderbird-2.0.14 client to report corrupt indexing, or worse not show the folder at next use.

Thanks.