If you’re thinking “It’s never too soon to kill Hitler,” well, forget it: you can’t go back in time and kill him.
On the other hand, there are other pests of European origin that possibly could be preempted:
DOCTOR: M. Sartre?
SARTRE: Mais oui!
DOCTOR: Jean-Paul Sartre?
SARTRE: (warily) Yes, monsieur, that is my name.
DOCTOR: The Jean-Paul Charles Aymard Sartre, the existentialist philosopher, playwright, novelist, screenwriter, political activist, biographer, and literary critic, one of the leading Figures in 20th Century French philosophy? (Ed: the Doctor reads Wikipedia.)
SARTRE: (Amazed) I am?
(The Doctor then raises the cricket bat and brings it down on Sartre’s head. Sartre falls out of his chair and sprawls inert on the pavement.)
DOCTOR: Not any more.
(The door to the Tardis opens. A young girl with long brown hair, wearing jeans and a sweatshirt with the initials CUNY steps out.)
GIRL: Is it done?
DOCTOR: (Looking down at Sartre.) Yup. When he wakes up in the hospital he’ll have forgotten everything he learned at the Sorbonne. He’ll decide to go into chicken farming in Provence. Being and Nothingness will never be written.
GIRL: Oh thank God!
Nominations for subsequent whacks by the Doctor will be received below.