SpamAssassin is just a great tool to block the incoming spams to your server. However, you might just need some other tools in addition to SpamAssassin, if you feel lotta spams are making it past SA.
Two of my suggestions from WHM are :
1. Enable the RBL option from WHM , which will check each of the IPs that is attempting to send mails to your server and if the IP is in the RBL list , it will automatically block the mails.
This option can be found from WHM as follows :
Home »Service Configuration »Exim Configuration Manager>> RBLs>>Manage Custom RBLs
You can enable the in-build lists or add your RBL lists from above.
2. Enable the setting to Reject SPF failures , which will reject emails from domain names without valid SPF records.
You can enable this option from :
WHM » Service Configuration » Exim Configuration Manager>>ACL
These two tweaks are seen as very effective when it comes to reject unwanted spams from hitting your server !
Still getting incoming spam ? Then you might need to think of training SpamAssassin. You can check this post here on more info on that.