You can gently lift it off excess with a soft brush. Hold the work at an angle and lightly bush/flick off the surface pastel.You say that the pastel can be worked over and over again, so should I attempt to remove all detail first and if so what is the best way to do this without making a horrible mess??
Or tacky it off with blue tack. I have even heard that someone used sticky tape, but I have never tried it and would be afraid to.
Once you have removed a bit of the pastel (don't try to remove all) you can then draw over it again.
If you don't want to remove the pastel, you could try covering it with really soft pastels like Sennelier (rembrants are too hard)