[Dovecot] Disabling access for one user

[Kenneth Porter]

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?