[Dovecot] disabling mail delivery for a user

How can I disable mail delivery for a user yet still allow access to their mailbox from another account which their mail is shared to?

If I put them in a userdb which has "deny = yes" that just disables their ability to login, yes? And they would still have mail delivered.

I want to disable their ability to login AND bounce all mail delivered to them, preferably solely through dovecot configuration. I am using lda, obviously, for mail delivery. (I could configure the MTA to reject mail for them if there is no other option.)