If you banish someone or ignore him, you ostracize him. When the Iranian president claimed that the Holocaust was a hoax, he was ostracized by the international community.
Ostraka is an ancient Greek word for pottery shard. Thousands of years ago, in the Greek city of Athens, there was a public process where you would write the name of someone you wanted to kick out of town on a broken ceramic fragment. If enough Athenians wrote the same name, that person was sent away for ten years. This process was called an ostracism.