How to stop or reduce graphite from reflecting light?
Spray fixative should make the drawing more matte as well as protecting it. 

You could also try a layer of acrylic matte medium, sprayed or brushed, but that would be a little more involved and riskier to the original work if it cannot handle the wet medium. 

Either way do some test pieces before trying on the finished work. 

I also would recommend drawing with a softer lead for dark areas, it will require less working and less material to get the tone you want, which means less buildup and less gloss.