What are the ways that people convince a mobidly obese person to stop eating high sugar high fat high sodium foods?

Self motivation: The best way for them to stop is if they want to stop doing it themselves. If they are not self-motivated or ready, your "convincings" will be less effective. Other possible ways is to show them the ramifications of their actions, or if something "bad" happen to them that will make them wake up. Also, we recommend slow weaning or just cutting down instead of stopping the above food completely.

Convincing or maybe: The person is convinced but just can't seem to do it. That's more likely. Why that is so is still not clear. Even in some folks who do stick to a low sugar, low fat and low sodium diet don't lose weight. That is in part because they don't reduce total calories to a point where they lose weight. Probably most important of all is getting enough exercise so metabolism can accelerate & so lose weight.