He made one mistake. You can't write him off completely because of it.
Tips
usage
Colloquial and vivid. Typically used in the negative: 不能一棍子打死, 不要一棍子打死, arguing against overreaction to a single flaw. Common in management, criticism, education.
memory
Picture one 棍子 (stick/club) swinging down to finish the argument. Overkill, hence the negative usage warning against it.