How to Blacklist Email Address in Direct Admin (Spam Assassin)
Login to Direct Admin console and click on your target domain.
Click on Spamassassin Setup as highlighted below:

Scroll down and fill in the email under Email Blacklist section. Enter the full list of emails which you intended to blacklist as seen in the example below:

NOTE: To block any incoming email from the specific domain example : blacklistdomain.com please insert the format *@blacklistdomain.com
Press the Save button to confirm the final changes.
Click on Spamassassin Setup as highlighted below:

Scroll down and fill in the email under Email Blacklist section. Enter the full list of emails which you intended to blacklist as seen in the example below:

NOTE: To block any incoming email from the specific domain example : blacklistdomain.com please insert the format *@blacklistdomain.com
Press the Save button to confirm the final changes.
Updated on: 06/03/2020
Thank you!