One particularly infamous and long-lasting anti-Semitic canard is the "blood libel", the idea that Jews commit acts of human sacrifice and consume the blood of the victim (never mind the fact that blood, human or otherwise, isn't kosher). Dating back to at least the 1st century BCE, the blood libel has long been used as an excuse to persecute Jews, and unfortunately still crops up from time to time.
Your challenge is to prevent the idea of the blood libel from ever being widely accepted.
Your challenge is to prevent the idea of the blood libel from ever being widely accepted.