How does one respond to those who deny the legitimacy of the Izar and mock the Sunnah?

It appears that the esteemed questioner, by "legitimacy of the izar," is referring to the hadiths that prohibit izbal (letting one's garment hang below the ankles), among them the saying of the Prophet, peace and blessings be upon him: "There are three whom Allah will not speak to on the Day of Resurrection, nor will He look at them, nor will He purify them, and for them is a painful torment." And he mentioned among them: "The one who lets his garment hang low." And his saying: "Whatever is below the ankles of the izar is in the Fire."

If the man intends mockery by what has been narrated from the Prophet, peace and blessings be upon him, then he is a disbeliever. And if he is ignorant, then explain to him what is authentic from the Prophet, peace and blessings be upon him, and be gentle with him.