The Jargon File (version 4.4.7, 29 Dec 2003):

kill file
 n.

    [Usenet; very common] (alt.: KILL file) Per-user file(s) used by some 
    Usenet reading programs (originally Larry Wall's rn(1)) to discard
    summarily (without presenting for reading) articles matching some
    particularly uninteresting (or unwanted) patterns of subject, author, or
    other header lines. Thus to add a person (or subject) to one's kill file is
    to arrange for that person to be ignored by one's newsreader in future. By
    extension, it may be used for a decision to ignore the person or subject in
    other media. See also plonk.


The Free On-line Dictionary of Computing (30 December 2018):

kill file

   [Usenet] Per-user file(s) used by some Usenet reading
   programs (originally Larry Wall's rn) to discard summarily
   (without presenting for reading) articles matching some
   particularly uninteresting (or unwanted) patterns of subject,
   author, or other header lines.  Thus to add a person (or
   subject) to one's kill file is to arrange for that person to
   be ignored by one's newsreader in future.  By extension, it
   may be used for a decision to ignore the person or subject in
   other media.  See also plonk.

   [Jargon File]