[Dovecot] disabling mail delivery for a user

[Frank Cusack]

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.)