I apologize, but I read your request backward. I thought you were wanting something being classified as spam to go to your inbox.
Other than moving from Inbox to Spam folder like 1000 more times to get sa-learn to understand you don’t want email from this dufus, you can edit one of the few files on the server that will not be overwritten with an update to MiaB.
Spamassassin will read any file in the path and format /etc/spamassassin/*.cf. You can create /etc/spamassassin/localrules.cf, or anything else that suits you, and put in the following:
blacklist_from username@example.com
This will add 100 points of spamscore to all messages from username@example.com, so every message from this address will end up in the spam folder.
Alternatively, if you do not even want this user in a spam folder, you can use the blacklist features of postfix, as described here:
But note that changes in that tutorial will be overwritten whenever MiaB makes a change to main.cf.
I haven’t figured out all of the options for us small server admins to declare email as spam to the various blacklists, but it is possible to declare it through dnswl.org. Also on that site you can add your server, which can help in some small ways to improve inboxing for your server and confirmed domains.
I actually find that the bulk of the worst spam to make it to my inbox comes from gmail, but good luck getting them listed at Spamhaus.