Hi
First you need to Login to WHM then select service configuration. You'll see the Exim configuration Editto option >> turn on SpamAssassin for all accounts.
To simply have the server delete and not deliver emails that are tagged as spam by SpamAssassin, You need to click on default option.
However, leave it disabled by default and enable it only for those clients who want to use it, because spamassassin generate load.
Thanx