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

LART
 //

    Luser Attitude Readjustment Tool.

    1. n. In the collective mythos of scary devil monastery, this is an
    essential item in the toolkit of every BOFH. The LART classic is a 2x4 or
    other large billet of wood usable as a club, to be applied upside the head
    of spammers and other people who cause sysadmins more grief than just
    naturally goes with the job. Perennial debates rage on
    alt.sysadmin.recovery over what constitutes the truly effective LART;
    knobkerries, automatic weapons, flamethrowers, and tactical nukes all have
    their partisans. Compare clue-by-four.

    2. v. To use a LART. Some would add ?in malice?, but some sysadmins do
    prefer to gently lart their users as a first (and sometimes final) warning.

    3. interj. Calling for one's LART, much as a surgeon might call ?Scalpel!?.

    4. interj. [rare] Used in flames as a rebuke. ?LART! LART! LART!?


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

Luser Attitude Re-adjustment Tool
LART

    (LART) Something large, heavy and painful, used to
   respond appropriately to particularly annoying lusers.

   The alt.sysadmin.recovery FAQ recommends the following
   LARTs.  A 2x4 works fine, but a real professional needs
   something a little more effective.  Unfortunately, this is a
   very personal thing, and no consensus has yet been reached on
   the group.  Everything from a simple, 7.65mm Walther (for the
   Bond fans only, it's not a very good gun) to a 155mm with
   depleted Uranium rounds has been suggested, some even going
   for exotic things like Thermite, nukes or flamethrowers.  For
   further info, look at the rec.guns home page.

   alt.sysadmin.recovery FAQ
   (http://ctrl-c.liu.se/~ingvar/asr/overview.html).

   (1998-12-09)