Turn on Spam Assasain, leave on level 5, this will either:

• automatically rewrite and add ***SPAM*** in to the subject line irrelevant of the rewrite notice at the bottom of the page. (once enabled goto account level filtering to set rewrite header etc.. ***SPAM***)
• deny the message before it even gets in to the que/postbox

Technical details of permanent failure:

Google tried to deliver your message, but it was rejected by the recipient domain. We recommend contacting the other email provider for further information about the cause of this error. The error that the other server returned was: 550 550 "The mail server detected your message as spam and has prevented delivery." (state 18).

make sure that your default address for unrouted email is set to fail no such user here (should be default to'no' on cPanel)

Test The anti spam setup

• create an email with this in the body of the email
  {code class="brush: text"}XJS*C4JDBQADN1.NSBN3*2IDNEN*GTUBE-STANDARD-ANTI-UBE-TEST-EMAIL*C.34X{/code}
• on 1 line only
• send from an external network

GTUBE test patterns found here: (the standard spam test)

http://spamassassin.apache.org/gtube/

These automatically score higher than 20.0 in Spam Assassin and are rejected immediately at SMTP time. The GTUBE test pattern scores 1000.00 on the spamAssassin Spam Score, scale so it would be automatically rejected.