First of all you should login to cPanel, go to SpamAssassin feature and enable it.
Then you will be able to:
enable or disable auto-deletion of spam feature (note that 'score' lower value is more aggressive)
enable or disable Spam Box (it will deliver any emails identified as spam by SpamAssassin into a separate mail folder named "spam")
configure Spam Assassin
Let's take a look at what we have there:
Blacklist allows you to specify addresses that should be blacklisted but tagged as non-spam. You should specify targets as ?email@domain.com, *@domain.com, or *.domain.com
Scores allows you to set the number of hits required before a mail is considered spam(note that 'score' lower value is more aggressive). In Spam Assassin, the scores are assigned using a neural network trained with error back propagation. This system attempts to optimize the efficiency of the rules that are run in terms of minimizing the number of false positives and false negatives.
Whitelist allows you to specify addresses that should be whitelisted but tagged as spam. You should specify targets as ?email@domain.com, *@domain.com, or *.domain.com