Preferences are handled in a confusing way:
a) Compiled-in preferences
b) Preferences in dspam.conf
c) Preferences of the the default user
d) Preferences of the user
in increasing order of importance.
Now, if there is any entry in the default users preference file, the entries in dspam.conf are skipped.
What happened to me: I tried to reduce the whitelist threshold to 10 and, unfortunately, did this by
dspam_admin add preference whitelistThreshold 10
After that, all my users got the dspam-signature in the body, because it was only set in dspam.conf.