I never tried taking Bishop's M off because I don't have any extra Bishop heads, but I have taken other facial features from other mates heads (eyebrows, pupils, whatever) with just a basic pocket knife. If you're careful, the scratch shouldn't be noticeable.
Yes, just last night, I removed the brown spots from Gollum's feet and legs! I used a pocket knife to get rid of most of it and then nail polish remover to get whatever was left