Peroxidase (POD)-like nanozymes have emerged as effective alternatives to natural enzymes owing to their stability and cost-effectiveness in biosensors. In particular, single-atom nanozymes (SAzymes) featuring Fe-N₄ active sites have attracted significant attention for their high catalytic performance. However, their 2D exposed active sites result in limited reaction selectivity and strong pH dependence, restricting their functionality under neutral conditions. This study introduces Ru-centered SAzymes coordinated out-of-plane with chlorine ligands (RuNC_Cl), achieving monofunctional POD-like activity. RuNC_Cl exhibited remarkable POD-like activity, which is 38-fold greater than its catalase (CAT)-like activity, indicating strong suppression of the competing CAT-like reaction. Density functional theory calculations and Bader charge analysis of RuNC_Cl reveal that repulsive forces preventing secondary H₂O₂ adsorption contribute to an increased energy barrier for the CAT-like reaction. This selective POD-like activity enables the precise detection of multiple biomarkers through a one-pot cascade reaction under near-neutral conditions. This advancement paves the way for the precise regulation of reaction pathways, enhancing the practicality of nanozymes for biosensing and related applications.