Disallow acces via imap, but keep lmtp running

[Julian Kippels]

Hi all,

what is the best way to temporarily disable access to a mailbox via
imap, but keep it possible to deliver to the mailbox via lmtp?
I want to migrate some mailboxes around and would like to ensure that
the users cannot access their mail while doing so.
I would like to keep the users from logging in entirely rather than
setting ACLs.

Thanks in advance
Julian