What you might try is this:
1) Install spamc / spamd from your linux distribution and make shure it is automatically started at boot time.
2) insert the path to the spamc program from your linux distribution in the file .spamassassin.rc instead of the ISPConfig spamc path.
This will enable spamd with your global spamassassin settings.
To use the settings from ISPConfig, you can try to create a symlink inside of the subdirectory .spamassassin that can be found in each users home directory like this:
ln -s ../.user_prefs .user_prefs