For the removal of automapping feature for Exchange mailbox, the manual method requires to remove full access permission to the mailbox and then reattach full access permission. While reattaching the permissions, the parameter for automapping will be changed to false (AutoMapping:$false parameter).

1. Remove the full access permission of the user from the mailbox.
2. Add back the full access permission level of the user for the mailbox but keep the automapping parameter to false.

To do this, follow the below steps:

1. Start Exchange Management Shell and run the following command:
   Remove-MailboxPermission -Identity UserMailbox -User UserName -AccessRights FullAccess -InheritanceType All

   Example –

   Remove-MailboxPermission -Identity Sales -User David -AccessRights FullAccess -InheritanceType All
2. Run the command:
   Add-MailboxPermission -Identity UserMailbox -User UserName -AccessRight FullAccess -Automapping $false

   Example –

   Add-MailboxPermission -Identity Sales -User David -AccessRight FullAccess -Automapping $false

The command will provide the full access rights of the mailbox Sales to the user David. But the automapping
feature is false. So, Outlook will not automatically add it.

Taken from 

https://www.nucleustechnologies.com/blog/remove-outlook-auto-mapping-for-a-mailbox-in-exchange/#:~:text=Remove%20automapping%20using%20Exchange%20Management%20Shell%201%20Remove,mailbox%20but%20keep%20the%20automapping%20parameter%20to%20false.