Take a chisel and a rubber mallet. Try to break some of the bark free. Get a pressure washer and go to work. As you see parts start moving stop and pry them up with your chisel. Cherry is my least favorite to clean second only to BAB. It really likes its bark and does not let go easily. I have a 2700psi pressure washer and it is still a chore. If no pressure washer I would stick it on a shelf and forget about it for a few years.....
Ohh, clean it whole. It takes almost as long to do a few pen blanks as the whole burl. All the bending down, clamping, washing, bending down flipping, washing, readjusting......blah blah blah. Do it before you cut it, trust me DAMHIKT