Objective based surface plasmon resonance microscopy (SPRM) is a label free technique, which is suitable for observing chemical and biological activities at metal-dielectric interfaces in real-time, with high resolution. Recently, digital holographic techniques have been combined with the SPRM to further increase its sensitivities. In SPRM and SPR digital holographic microscope (SPR-DHM) setups, cube beam splitters (BS) are commonly used to direct light onto the back focal plane (BFP) of the microscope objective. However, SPRs feature very strong absorptions, and the intensity of the unwanted reflections from the BS could be of the order of the SPR image intensity. These unwanted reflections will produce strong interference noise in both SPRM and SPR-DHM. The fringes thus generated have to be removed. The illumination being convergent, the reflections would focus in the direction opposite to the BFP. We propose to block them by inserting a 3D fabricated hard knife-edge filter set at that focus, in such a way as not to stop the SPR waves from forming images of interest. Experiments and simulations show that the filter can significantly improve the image quality by reducing the root mean square of the unwanted fringes from 12.6 to 0.500 in terms of gray levels.