I've added a filter to syslog-ng conf file. I think this should work.
Code:
# all messages from the auth and authpriv facilities
filter f_auth { facility(auth, authpriv);};
#filter the CRON messages
filter f_cron_msgs { not match("CRON*"); };
# auth,authpriv.* /var/log/auth.log
# + filter the CRON messages out
log {
source(s_all);
filter(f_auth);
filter(f_cron_msgs);
destination(df_auth);
};