What’s done can’t be undone, but at least you can keep it from happening again.

[Gedane zaken nemen geen keer, maar wel kan men verder voorkomen.]

Anne Frank (1929–1945) German-Dutch Jewish diarist

Diary (1944-05-07) [tr. Massotty (1995)]
    (Source)

(Source (Dutch)). Another translation:

What's done cannot be undone, but one can prevent it from happening again.
[tr. Mooyaart-Doubleday (1951)]