At what point should a country prohibit freedom of speech?

Never. As long as they know with that freedom comes responsibility. Many people don't understand that the right to speak freely is also one that warrants consequences and people that use words for hate or to spread intolerance either don't fully realize the consequence of their speech or simply don't care.

However, no one should be silenced, ever, no matter how ignorant, hateful, despaired, happy, introverted, provocative, insane, etc. etc....