interior_erase.rb

require 'set'

model = Sketchup.active_model
selected = Set.new(model.selection.to_a)
edges = model.selection.grep(Sketchup::Edge)
faces = Set.new(model.selection.grep(Sketchup::Face))

border = edges.reject { |edge| edge.faces.all? { |face| faces.include?(face) } }
selected.subtract(border)

model.start_operation('Interior Erase', true)
model.active_entities.erase_entities(selected.to_a)
model.commit_operation