That's possible, if i understood you correctly.

You mean, mail server continues accepting mails which delivered to this user, but this user cannot login through mail client or webmail, is it correct?

If so, you can easily achieve it with iRedAdmin-Pro, screenshots attached below:

Uncheck below items:
- Fetching mail via POP3
- Fetching mail via POP3S
- Fetching mail via IMAP
- Fetching mail via IMAPS

Check below items:
- Receiving mails for this account on mail server.

That's all.

If you don't have iRedAdmin-Pro installed, you can set in LDAP user object, or MySQL table "vmail.mailbox".

For LDAP, remove below values in user object:

- enabledService: pop3
- enabledService: pop3secured
- enabledService: imap
- enabledService: imapsecured

Make sure you have 'enabledService=deliver' in user object.

For MySQL, a SQL command should be enough:

$ mysql -uroot -p
mysql> USE vmail;
mysql> UPDATE mailbox SET enablepop3=0,enablepop3secured=0,enableimap=0,enableimapsecured=0,enabledeliver=1 WHERE username='user@domain.ltd';