Custom rules can be created for Spamassassin, I use a hand written list thats customised for my needs.
The location of the file is /etc/mail/spamassassin/98_myrules.cf
The file name is not important, the format of the file is as follows:-
body LocalStockRule /putcompanynamehere/
describe LocalStockRule Local Stock Name Block
score LocalStockRule 15.0
The above rule would look for a word in this case its putcopmpanynamehere and the spamassassin score would be increased by a score of 15.0