Ignore quarantine above a certain spam score?

Hi,

I was wondering if it is possible to silently discard mails when they
reach a certain (high) score from Spamasssassin?

$spam_tag_level = -999;
$spam_tag2_level = 5;
$spam_kill_level = 5;
$spam_dsn_cutoff_level = 15;

$final_spam_destiny = D_DISCARD;

This works quite fine for me, all mail with a spam score above 5 will
be quarantined to a special mailbox. However, many spam mails easily
reach a score of 30 or higher, so there is absolutely no point in
quarantining them, I would really like to silently discard them. Is
this somehow possible?

I'm using amavisd-new 2.2.1 / Spamassassin 3.0.2 with Postfix 2.1.5
on Debian Sarge.

Frank