Office365 Shared Mailbox - Stop incoming mail and send back an undeliverable message

I have a terminated user's email account in exchange that was converted to a "shared" mailbox and that user's email was set to forward to another user.  Now that months have passed, I would like to setup this mailbox so that anyone that sends an email to the terminated employee receives an "undeliverable message" - I know how to remove the forward but not sure about how to get the "undeliverable message" sent to anyone that sends this terminated employee an email.  Please advise how this can be accomplished on the terminated user's mailbox.  I did see a post from 2016 that addresses this issue somewhat but not sure if anything may have changed since that post.