That's the kill level, see the as/av tab in global settings.
zmprov mcf zimbraSpamKillPercent 50
(It's given in percentages-so that would kill anything with 10pts on the 20pt scale)
100% = 20pts
33% = 6.6pts
75% = 15pts
etc
You can change the action (discard vs bounce etc) in amavisd.conf.in (don't edit amavisd.conf directly, edit the .in and restart)
$final_spam_destiny=D_DISCARD;
You can also play with the dsn (delivery status notification) setting; so over a certain level you won't be responding 'I got your mail' to the spammers.
$sa_dsn_cutoff_level = 50;
To delete/not bother quarantining high scoring spam (therefore reducing the number of items in the quarantine) this setting allows you to discard quarantined spam above this level:
$sa_quarantine_cutoff_level = 90;
It is cleaned up every day though:
0 1 * * * find /opt/zimbra/amavisd/quarantine -type f -mtime +7 -exec rm -f {} \; > /dev/null 2>&1
Note: In that amavisd.conf.in file, wherever possible it's better to set the values with zmprov/admin console (ie: the tag & kill levels) so that it stays consistent across upgrades.
zmprov mcf zimbraSpamTagPercent 30
-would put everything above 6pts in the junk folder (and label with a custom **SPAM** in the subject line if you have that enabled) - I personally don't because it's already in the junk folder.
(It won't download via pop unless you point the pop at something other than the inbox, in some clients you can configure it like
user@domain.com%folder)