Well, you could reduce the roughness without removing it completely, it looks like it may not be rough enough. Are you using a map for your roughness or a scalar parameter?
Also though, I have found that the Sun in BP_SkySphere can cause reflections even on rough surfaces when using the planar reflection, maybe try reducing the brightness of the sun in BP_SkySphere (I think you have one in your scene), the variable is “Sun Brightness”.
Actually you could try adjusting the “Distance from Plane Fade End” value on your PlanarReflection, reduce it so that it only covers the lake