Abstract0D organic–inorganic metal halides (OIMHs) with intriguing luminescence encoded in their diverse crystal structure open wide opportunities for next‐generation optoelectronics. Yet, this structural diversity makes their precise synthesis challenging. Here, a facile yet efficient off‐stoichiometry antisolvent precipitation protocol is devised to synthesize pure Bmpip2PbBr4 (Bmpip = 1‐butyl‐1‐methylpiperidinium) polycrystals. Optical investigations reveal that the key to suppressing the by‐product, Bmpip9[Pb3Br11][PbBr4]2, generally occurring in a typical stoichiometric synthesis is to create a Br‐rich environment to promote the [PbBr4]2− formation while suppressing the formation of [Pb3Br11]5−. Moreover, this off‐sociometric protocol can be extended to the precise synthesis of Bzmim3SbCl6 (Bzmim = 1‐benzyl‐3‐methylimidazolium) polycrystals through the meticulous control of [SbCl6]3− formation in the solution. The resulting Bzmim3SbCl6 polycrystals show a nominal light yield of 24600 photons MeV−1, which is 6.8 times higher than that of its by‐product, namely Bzmim2SbCl5, and outperforms that of commercial LuAG:Ce. As a result, the scintillators made of Bzmim3SbCl6@PMMA achieve a decent spatial resolution of 8.3 lp mm−1. This work highlights the importance of regulating the metal polyhalide intermediates in precisely synthesizing 0D OIMHs.