I had that same problem with a plastic dust cap on a Solida crank on a 1985 Peugeot. I used an x-acto knife's smallest chisel -style blade to slowly knock our pieces. Eventually got enough out to where I could get my socket on the bolt sufficiently to get the bolt out. With the bolt out I was able to get out the rest of the plastic. Was concerned that I was going to damage the threads on the crank arm so that the puller threads wouldn't engage, but that did not happen--the plastic pieces down in the threads seemed to protect them. For a derailleur cable end, I used a tiny dremel drill to get enough of that out to where I could remove it. BTW, great looking bike for 40 bucks!