To add a rake task to the rails application which removes all the image and audio files created by captcha, create a file named remove_captcha_files.rake in the lib/tasks directory.
Add the following code to the lib/tasks/remove_captcha_files.rake file…

desc "Remove captcha images and audio files"
task :remove_captcha_files do
  image_path = "#{RAILS_ROOT}/public/images/captcha/"
  audio_path = "#{RAILS_ROOT}/public/captcha_audio/"
  Dir.foreach(image_path){|file| File.delete(image_path+file) if (/^.*.jpg$/).match(file)} if File.exist?(image_path)
  Dir.foreach(audio_path){|file| File.delete(audio_path+file) if (/^.*.wav$/).match(file)} if File.exist?(audio_path)
  puts "Captcha files removed."
end

You can confirm that task is added to your app by running
rake –task

To remove all the files created by captcha, simply run the command
rake remove_captcha_files
from the command line from your application root.
To better view the code visit here