Hm, looking in amavisd.conf, there's some lines in there that might be of interest
(Values taken from my install.)
$sa_tag_level_deflt = -10.0; # add spam info headers if at, or above that level
$sa_tag2_level_deflt = 4.4; # add 'spam detected' headers at that level
$sa_kill_level_deflt = 15; # triggers spam evasive actions
$sa_dsn_cutoff_level = 15; # spam level beyond which a DSN is not sent
$sa_quarantine_cutoff_level = 15; # spam level beyond which quarantine is off
Note $sa_quarantine_cutoff_level could take a different value from $sa_kill_level_deflt
I wouldn't really recommend messing with these, but if you're prepared to spend time researching how spamassassin, amavisd-new, and Zimbra interact with each other (or if you don't care too much what happens to your mail while you're experimenting) you might get somewhere.
But also note that amavisd.conf is rewritten whenever you restart Zimbra. The proper place to make changes is amavisd.conf.in. There, both variables are set equal to zimbraSpamKillPercent. So you'd need to make the change there, removing the reference to zimbraSpamKillPercent, to make it permanent.