Create a blacklist

Silly me. Just found out about this page:

http://www.postfix.org/addon.html

... and I quote:

mailscanner system, works with Postfix and other MTAs. WARNING: This software uses unsupported methods to manipulate Postfix queue files directly. This will result in corruption or loss of mail. The mailscanner authors have sofar refused to discuss a proper access API or protocol.

Case solved. amavisd-new, it will be.

Thanks for everyone who have tried to answer!

Edit:

In the comments, @Spikolynn referred to http://wiki.mailscanner.info/doku.php?id=documentation:configuration:mta:postfix:politics , which explains how Postfix now implements a 'holding' queue which should prevent queue corruption.

Many improvements have happened on the Postfix side since this question was asked; so if you happen upon this Q&A as a result of a search, please be aware that the situation might not be as clear cut as it was when I came upon the problem.

You should put this options on your /etc/amavis/conf.d/50-user file:

$sa_tag_level_deflt  = -999; # add spam info headers if at, or above that level
$sa_tag2_level_deflt = 6.2;  # add 'spam detected' headers at that level
$sa_kill_level_deflt = 6.9;  # triggers spam evasive actions (e.g. blocks mail)
$sa_spam_subject_tag = '**Spam**';

Configure the SPAM confidence level to the ones accordingly to your needs.

If you don't want to manage your score levels, you can set the quarantine directories to undefined ones. To do this add those lines in the same file:

$clean_quarantine_to      = undef;      # local quarantine
$virus_quarantine_to      = undef;      # traditional local quarantine
$banned_quarantine_to     = undef;      # local quarantine
$bad_header_quarantine_to = undef;      # local quarantine
$spam_quarantine_to       = undef;      # local quarantine

If you want to check the information or take a look at additional information here's the Quarantine block on the AMaViS website: http://www.amavis.org/amavisd-new-docs.html#quarantine