In the area of bluff body aerodynamics, controlling the vortex shedding is of great importance to prevent damages caused by the vortex-induced forces. Accordingly, a short and thin splitter plate has been utilized at different downstream positions to interfere with the vortex wake of a confined square cylinder in the framework of an optimal passive control study. Vorticity-stream function formulation and finite-difference method have been used to numerically simulate the two-dimensional laminar flow. For preventing the flow from becoming three-dimensional, the Reynolds number is kept below 250. Validation of the code and mesh dependency analysis are offered. Based on the flow patterns acquired, the optimal position of the splitter plate behind the cylinder can prevent the detachment of the pair of vortices from the cylinder and will keep the streamlines symmetric around the zero streamline. Consequently, vortex shedding does not occur and the vortex-induced forces are eliminated entirely. Interestingly we showed that the vortex can be suppressed even when the splitter plate is asymmetrically arranged behind the cylinder, especially in the case of positioning the body close to the surrounding walls.