There isn't a way to disable mailbox migrations completely, but the Protocols page has a setting - Max Mailbox Migration Threads - that you may find helpful. This setting restricts how many mailbox migrations can run simultaneously. If the server reaches that limit, any additional migrations will be added to a queue and will wait until a thread becomes available. Lowering this setting should help you keep the load on your server down.
Andrew Barker
Software Developer
SmarterTools Inc.
www.smartertools.com