You may increase the score for the blacklist symbols to “drop” the spam completely.
In the rspamd UI go to symbols and search for “blacklist”:
Set a high score for FROM_DOMAINS_BLACKLIST and FROM_SUBDOMAINS_BLACKLIST, in my case 20 because I set the “Deny message spam threshold” to 20 and spam higher than 20 will be dropped.
This will drop all spam of blacklisted domains and subdomains.