Well, seek higher religious authority to deal with this guy.
An Oracle responds to your plight, and travels from a secluded monastery to your court. Clad from head to toe in thick black robes adorned with multicoloured jewellery, he bows before you, and requests that you assemble a court, where all will testify before the gods as to the nature of the suspect.
Your throne room is transformed. Drapes are hung before the windows, the torches burn low, incense wafts through the air. Lines of... let's call it 'paint' criss-cross the floor, forming intricate, mystical patterns. Golden idols are placed at strategic points, and a choir of acolytes stand in the shadows, chanting softly. The Oracle himself sits cross-legged beside the throne, piercing eyes glimmering beneath folds of cloth.
The terrified herbalist is brought forth, chained, and sat on the other side of the throne. A small incision is made in his palm, and red blood, vivid in the strange lighting, spills forth.
Local priests, villagers, wise men, and any others whose testimony might be relevant enter the court reverently. The Oracle extracts from each an oath of truth, and has them sit.
The court lasts several hours. The Oracle calls people forth, and has them speak. Fear and faith compel the truth out of them. On occasion the Oracle will ask a piercing question, reducing witnesses to stammering wrecks.
Eventually the Oracle bids all be silent. The chanting ends, and the air itself hangs still in anticipation.
"Let it be known before the gods", speaks the Oracle softly,
"I have heard, I have seen, and I have felt. The truth is revealed."[1,4,3,2]
"The man is a vile sorcerer, of evil intent and way. He has performed magics to hide the truth, but this court has revealed it. His death will please the gods. His soul shall be sealed in this shell, and cast into the sea, where it shall trouble only the fish"The herbalist- no, sorcerer- screams in agony as a knife is plunged into his throat. He struggles against his chains, even as his blood spurts through the air, drenching those in the front rows. The Oracle stands and holds the soul-shell before the wound, filling it with blood, then hits the man's forehead with the flat of his palm, whereupon he crashes to the floor and is still.
The court empties without a word. The Oracle's assitants clean the floor and dispose of the body and shell, and politely collect their payment of five crowns from the steward.
An Oracle holds court, and judges the Herbalist guilty. Five crowns are secured as payment for the service. If this does not please the people, nothing will.