If you want to disable diginotar's root CA on your Mac (for Safari/Chrome) you can open Keychain Access, select the "System Roots" keychain at the top left, find the diginotar certificate in the list, and delete it (or disable it, which is what I did).
EDIT: This definitely works for Safari, not 100% sure if it does for Chrome after all.
EDIT: This definitely works for Safari, not 100% sure if it does for Chrome after all.