Definition
After destroying or devastating something, to do something additional to make sure it is permanently destroyed, as a precaution or out of spite.
How People Actually Use It
A practice in ancient warfare where conquerors would deliberately spread salt over the lands of a defeated enemy, rendering the soil infertile and unsuitable for farming. While traditionally intended to refer to someone who is genuine, trustworthy, and of great worth or reliability more accurately describes one who is nothing but a toxic individual.
"I noticed having you in my life was to salt the earth"
Source: Urban Dictionary