[Dovecot] Restrict clients access during mailboxes convertion ?

Hello

Is there a way to restrict access to Dovecot server during mailboxes convertion ?

I need dovecot to be running to convert mailboxes from a format to another, but I would like to block all our LAN clients until the end of convertion.

If there is no internal Dovecot mechanism I will use some firewall rule on the server.

Thanks