I have run into this problem with my woodworking endevours. Chemicals (acetone or methylene chloride) will do the trick, but will bugger up the adjacent finish on the solid stock to which the plastic wood (cellulose) is appended. I found that a Dremel high speed router will easily remove the plastic wood. Go slowly so as not to overheat the bit (get plastic fouling). Use safety glasses as this stuff is nasty to the eyes.

Iggy