Disallow acces via imap, but keep lmtp running

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