Stop INCOMING mail without disabling the account

I run exchange 2013 CU3 on windows 2008R2 - I several people who are gone, their mailboxes need to be accessible to other people in the department. When these people departed they never unsubscribed to personal listserv and are still receiving personal email to their mailbox.

My question is: Is there a way to stop all future incoming mail (internally & externally) without disabling the account, like I said these mailboxes have to be accessible.

Any help would be appreciated.

RJM