404-to-start is a plugin by that can redirect all 404 page not found errors permantly (or temporarily) to the start page or any other site/page you like.
This will help you solve problems with external bad links which are not under your influence.
Chose between 301 – Moved permanently (default) and 302 – Moved temporarily.
Plugin homepage; http://1manfactory.com/wordpress-plugin-404-to-start-send-every-page-not-found-error-to-start-page/
Download: http://wordpress.org/extend/plugins/404-to-start/