Sayyiduna Ali (may Allah be pleased with him) said that if a person calls another 'fasiq' (sinner) or 'khabith' (wicked), there is no fixed hadd (prescribed punishment) upon him; rather, the ruler should punish him by way of ta'zir (discretionary punishment) as he sees fit.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحدود / 17149