Tafsir of Al-Ma'aarij 70:15

Nay! is a refutation of the criminal’s desire [to ransom himself], and an explicit statement regarding the impossibility of salvation. The pronoun in it is refers to the Fire, indicated by the mention of "the punishment" [in the preceding context].

His saying, the Exalted, a blazing flame is the predicate of inna (indeed). It is a proper name for Hell, or for the second level of its levels, derived from al-laza, meaning pure flame. It is prevented from declension (tanween) due to being a proper noun and [its characteristic of] femininity. It is also permissible that it is intended to mean "flame" as an intensive [hyperbolic] expression, as if the entirety of it were pure flame. The omission of the tanween is either to treat the juncture as a pause, or because it is a generic proper noun equivalent to a form containing the definite article al-, similar to sahar (when one intends a specific magic).