Trying out your solution here, in my own local config of spamassassin I could setup RBL checks to further add score to incoming spam and more effectible filter out incoming spam, I was using the following RBLs checks:
zen.spamhaus.org
cbl.abuseat.org
dnsbl.sorbs.net
bl.spamcop.net
b.barracudacentral.org
psbl.surriel.com
spam.spamrats.com
query.senderbase.org
truncate.gbudb.net
in your control panel I cannot seem a way to do this , unless I missed something can you provide some guidance on this? Is this not offered?