[Dovecot] Disabling access for one user

I'm converting my system from mbox to Maildir one user at a time, and I'd like to disable access for the user I'm working on. Is there a way to tell Dovecot to lock out a specific user? Perhaps with a control file in the home directory?

At the same time, how can I get sendmail+procmail to tempfail incoming messages for one user until I'm done?