You can use SPAM filters to block emails from specific email addresses.
Log in to your DirectAdmin Account.
In the E-mail Manager section, click on the SPAM Filters icon or type SPAM Filters in the navigation filter box, and the option will appear. Click on it.
Under Block By from the drop-down menu, choose the Email option.
In Value, type the full email address you wish to block.
If you want to block more email addresses, repeat this tutorial from steps 3 to 5.