。
She absolutely refused to let him into the room.
Literal
She [topic-は] no-matter-what him [object-を] room [into-に] let-in [refused to-ようとしなかった].
入れる is the transitive 'put/let in' (intransitive: 入る 'enter') — here meaning 'admit, allow in.' Pairing with ~ようとしない turns it into 'wouldn't even try to let in,' framing a deliberate, sustained refusal. The に on 部屋 marks the destination of the entering. Watch the transitive/intransitive pair: 入れる ('let X in') vs 入る ('go in / enter') — choosing the wrong member rephrases who is doing what to whom, a high-stakes detail in Japanese verb morphology.