Changing the SA config

Hi guys,
          I am very new to linux and exim so I apologise for any massive 
noobness. I've setup a server using ubuntu 8.04 server, exim4, dovecot, 
clamav and SA. All seams to be working well and nicely I've sent mails 
that contained test virus signatures and would flag as spam and this 
worked (well spam headers/report were added). I then wanted to change 
SA's config to alter the subject of spam mails.

To do this I edited /etc/spamassassin/local.cf to have rewrite_header 
Subject *****SPAM***** I then promptly restarted the spamd and exim4 for 
good measure and tried again. Again the message got the headers flagging 
it as spam but the subject of the message was not changed. So that didnt 
work so I then tried doing exactly the same thing with my 
/mail/.spamassassin/user_prefs to exactly the same thing. Restarted the 
daemons tried again, once again the mail had the spam headers but the 
subject isnt changed. So what I would ask is where on earth do I edit this?

Thanks in advance guys.