A judgment can be refuted, but never a prejudice.

[Ein Urtheil läßt ſich widerlegen, aber niemals ein Vorurtheil.]

Marie von Ebner-Eschenbach (1830-1916) Austrian writer

Aphorisms [Aphorismen], No. 4 (1880) [tr. Scrase/Mieder (1994)]
    (Source)

(Source (German)). Alternate translation:

An opinion can be controverted; a prejudice, never.
[tr. Wister (1883)]