I used this hint now.
It saved my day as my mashines where going down several times having loads of 100% and hundreds of clamscan processes running ...
After changing to clamd/spamd I have not noticed any higher load any more.
So kudos to Croydon.
If there will be an upgrade to ISPconfig I think I only need to do the following steps:
sed -i -r "s/\| .*\-\-prefs\-file.*/\| \/usr\/bin\/spamc/" /root/ispconfig/isp/conf/spamassassin.rc.master
Or do I need to do more. But I think an ISPCONFIG2 update only touches /root/ispconfig. Or am I wrong with that?