What is the difference between "haraam" (forbidden) and "la yajooz" (not permissible), and is one who abandons an emphasized Sunnah considered disobedient?

There is generally no difference between something being forbidden (haram) and something being impermissible (ghayr ja'iz). Permissibility (jawaz) means Mubah (permissible), which is the choice between performing an act and refraining from it. If this choice is absent, then the thing becomes forbidden or impermissible. Some jurists may use "impermissible" to denote invalidity, insufficient fulfillment, or something disliked (makruh). As for the emphasized Sunnah (Sunnah Mu'akkadah), an individual who abandons it is not described as disobedient. However, consistently abandoning it is considered a breach of uprightness (khawariq al-muru'ah). Furthermore, it is not permissible for the people of a town to collectively agree to abandon certain Sunnahs, such as the Adhan.