Those guilty of a serious crime who refuse to reform must be severely punished.

guilty of a serious crime为形容词短语，作后置定语修饰those (people).

who refuse to reform为定语从句，修饰Those guilty of a serious crime。其中reform为不及物动词，意为改造。

那些犯了重罪又拒绝改造的人，必须受到严厉惩罚。

Yet no firm evidence had come to light that the men arrested were actually responsible.

that the men arrested were actually responsible这是that引导的同位语从句，修饰主语evidence。为了避免头重脚轻，that从句后移到谓语之后，构成了分隔式修饰。

句子大意为：但没有确凿证据证明被逮捕的那些人实际上就是罪犯。