Move SPAM to Junk folder and mark as read

Using Dovecot Sieve, you can easily move all spam to a Junk folder, and mark the spam messages as read:

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if header :contains "X-DSPAM-Result" "Spam" {
  setflag "\\Seen";
  fileinto "Junk";
  stop;
}

I generally define this rule globally for all users. I use DSPAM, but this can easily be adapted to other spam filtering tools, provided they add extra headers in the filtered mail.