To enable quarantine support simply change the following line in
/etc/amavisd/amavisd.conf:

  $QUARANTINEDIR = undef;      # -Q

to

  $QUARANTINEDIR = "$MYHOME/quarantine";      # -Q

then restart amavisd.

The following will make the change for you:

  sed -i.orig -e '/\$QUARANTINEDIR =/s,undef,"$MYHOME/quarantine",' \
      /etc/amavisd/amavisd.conf

PLEASE NOTE:  This is intentionally a manual step.  Before turning on
the quarantine feature of amavisd-new, be sure that there is a
sufficient amount of free space on /var/spool/amavisd/quarantine.
"Sufficient" depends entirely on your environment.

In the default configuration, files in the quarantine directory will
be automatically deleted by tmpwatch after 30 days.  See
/etc/cron.daily/amavisd to adjust this behavior for your environment.