You can do this two ways.
a. Enter your cPanel control panel / Email Management Tools then select SpamAssassin
b. Next, confirm Spam Assassin is ENABLED
c. Then, click Configure Spam Assassin
d. One the following page, add the email you wish blocked after: blacklist_from
e. You may also white list email addresses here as well
a. Enter your cPanel control panel / Email Management Tools then select either User level or Account level filtering. User level filters email for a single user while Account level filters email for the entire account (domain),
b. Follow the instructions to add any email that you want blocked.