You should simply remove your full mailbox access on the other mailbox using exchange management console or Exchange Management shell.
The utility that you are asking about is MFCMAPI but it is not less than dangerous to use it without proper knowledge. I would not recommend you removig these mailboxes using MFCMAPI. A few of places you can check instead:
1. Remove yourself from Full Mailbox Access permissions of the additional mailbox.
2. Check msExchDelegateListLink attribute on additinal mailbox doesnt not contain your user account’s DN. If it contains it then simply remove it.
3. Wait for AD replication to complete.Article ID: 32, Created On: 6/7/2011, Modified: 1/13/2013