Why don't you try doing it in Exim since you must be having access through WHM do all edits to configuration through WHM so that your customization is not overwritten during subsequent Exim Upgrades or cPanel upgrades.

You could set your exim to reject all Obvious SPAM (say score of 20+) to reject at SMTP level which would also mark all mail as SPAM depending upon the score that you have set in server wide .cf file.

In case you would like to do then follwoing could be done:
Login to WHM and Click on Service Configuration>Exim Configuration Editor In Advanced Mode.

In the top box... Add the following line:

spamd_address = 127.0.0.1 783

Scroll down to ACL section (a set of three text areas). In the middle one, Add the following:

Just before the last line Add following (last line is accept)

##### SpamAssassin check #####
warn message = X-Spam-Score: $spam_score
spam = nobody:true
warn message = X-Spam-Report: $spam_report
spam = nobody:true
warn message = Subject: *** SPAM *** $h_Subject
spam = nobody
deny message = This message scored $spam_score spam points. Sorry we do not allow Mails Which Are SPAM .
spam = nobody:true
condition = ${if >{$spam_score_int}{200}{1}{0}}
###Spam Assassin Check End###

Scroll right down and hit on
Save

Above config's will always remain there despite all upgrades ... You could modify the default score of 5 to whaever you feel fit ...

I presume that you are Not having mailscanner+clamav combination. In case you are then switch to exiscan+clamav combination which would reduce the load and also reject all Virus infected messages at SMTP level...

783 is the port where spamd is listening. In case it's confugured on some other port, you could do a check by running netstat -nlp and see which port spamd is on...

HyTC.