What is the ruling on saying: "Do not wear hijab if your hijab is like this" to women who consider hijab a mere formality and show the front of their hair?

Your statement that covering part of the head is less severe than uncovering all of it is incorrect. Rather, women must be taught the correct description of the hijab, which covers the entire head and hair, by agreement. It should also be unadorned and not attention-grabbing, because the purpose of the hijab is concealment. Let enjoining good and forbidding evil be done with wisdom and good admonition, and in a situation where there is no fear of temptation.