Please edit the scores as I recomended above, this will give you already a spam recognition rate of about 95 - 99% without false positives and the spamfilter will get better day by day as it uses auto learning. There is no need to add any further tweaks and rbl's are automatically used. To keep spamassassin up to date, just run regularily the command:
and then restart amavisd.
ISPConfig is not using spamassassin directly, it uses amavisd which loads the spamassassin library. For details on amavisd-new, take a look here: