Luser Attitude Readjustment Tool, traditionally a blunt weapon, but more often than not this refers to an emailed complaint of abuse or the action of sending that complaint. (BOFH)
Acronym for Luser Attitude Readjustment Tool, meaning to adjust the attitude of a spammer by bringing them to the attention of their ISP. To LART someone is to file complaints against them.
Short for Luser Attitude Adjustment Tool, as in "The spammer was sharply LARTed right away and lost his account."