The next big step for fixing spam issues is to build a robust header_check and body_check file for Postfix.
Thats implemented in ispconfig, so do not add such a file manually to ensure that you will not get conflicts. Use the header and body filter function from ispconfig instead.
By the way, I wont use such filter files for spam filtering. The normal spamassassin and blacklists filter already about 99.x % of all spam and adding such manual filters will just rise the risk of false positives. You can not win the battle against sapm with manual filter rules.