The Zodiac Killer’s 32-symbol cipher (Z32) has long been treated as an unsolved or poorly executed cryptographic message, most often hypothesized to encode geographic coordinates or navigational instructions anchored at Mount Diablo. Despite decades of attempted solutions, no decoding has yielded a stable, reproducible result. This paper demonstrates that the failure to decode Z32 is not incidental but structural. By formalizing the coordinate hypothesis algebraically, enumerating symbol-to-parameter degrees of freedom, and evaluating all admissible mappings consistent with the Zodiac corpus, the paper proves that no determinate decoding exists within the defined solution space. Attempts to resolve the cipher through constraint addition, grid embedding, rotation, or dial adjustment either preserve underdetermination or produce overdetermination and internal contradiction. The analysis further proves that this ambiguity is invariant under all admissible transformations. On this basis, the paper introduces and formally defines the concept of an anti-cipher: a symbolic construction that resembles a cipher in form while encoding a terminal state rather than recoverable content. Z32 is shown to satisfy the necessary and sufficient conditions for this classification. When analyzed in conjunction with the earlier Z13 cipher, Z32 is shown to function as a terminal operator that irreversibly closes the identity-disclosure pathway opened by Z13. These conclusions follow strictly from constraint analysis and algebraic reasoning, without reliance on psychological interpretation or speculative intent.