What is a Self-cancelling Word™?

_{This is in the spirit of the What is a Word/Phrase™ series inaugurated by JLee with his original Phrase™ and Word™ puzzles.}

If a phrase adheres to a certain rule, then I call it a Self-cancelling Word™.

Use the examples below to find the rule.

In case you want it in CSV:

Self-cancelling Words™, Not Self-cancelling Words™
WARM, HOT
CUSTOMIZE, PERSONALIZE
STOPPED, BLOCKED
PICTURE, IMAGE
FRIGHTENING, SCARY
ARRANGEMENT, LAYOUT
HIGHWAY, FREEWAY
PHOTOGRAPHER, PAPARAZZI
NUMEROUS, MANY
FAILING, DECLINING 
BOXING, BATTLING
OVER, ACCOMPLISHED

The puzzle relies on the series’ inbuilt assumption, that each word can be tested for whether it is a Self-cancelling Word™ or not on its own.

Hint :