You shouldn't see much spam with those settings, I don't use any of those I've just changed smtpd_reject_unlisted_recipient in the zmmta.cf file to 'yes'. and my tag/kill are set to 25/66. In current versions of Zimbra we have disabled DPSAM by default as there was a performance problem on larger sites, I've also re-enabled that.
The lifetime of mail in the Junk folder is controlled by the 'lifetime' option in the admin ui on the COS/Advanced tab. There is an article
here about using RDJ with Zimbra, I did use it a while back but have since discontinued it.