Provavelmente você precisa definir $ QUARANTINEDIR para esvaziar em amavisd.conf
# Location to put infected mail into: (applies to 'local:' quarantine method)
# empty for not quarantining, may be a file (Unix-style mailbox),
# or a directory (no trailing slash)
# (the default value is undef, meaning no quarantine)
#
$QUARANTINEDIR = '/var/virusmails';