/usr/share/doc/debian/bug-reporting.txt.gz
says I can add an X-Debbugs-No-Ack pseudo header
and that's what I did in
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1027691
but as you see an acknowledgment was sent anyway:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1027691;msg=4

OK, my experiments show, at least for the pseudoheader, the document
needs to mention:


   an X-Debbugs-No-Ack header or pseudoheader in your e-mail (the contents
   of this header do not matter). If you report a new bug with this

                               ^, but they cannot be empty!

   header, you will need to check the web interface yourself to find the
   bug number.