If you just want to remove the color from the World Environment, but keep the lighting strength in B&W, place a RGB to BW Node between your HDRI texture and the Background Node of your world shader:

If you want to keep the HDRI colors, but still have only white lighting, mix a regular background with the black and white background, using the "Is Camera Ray" output of a Light Path node as the mix factor:

If you want to remove all reflections from the HDRI and replace it with pure white, mix the Environment Background with a plain white Background, using either "Is Reflection Ray" or "Is Glossy Ray" (depending on your needs) from the Light Path Node as a mix factor:
