Is it permissible to lie by spreading rumors about the presence of jinn in a canal to deter farmers from throwing trash into it, especially since this method has proven effective with one's wife?

Lying in this matter is impermissible for two reasons: First, lying is unlawful and not a characteristic of a Muslim, unless it is out of necessity or for a recognized benefit that does not cause harm to others. The Prophet, peace and blessings be upon him, said: "Adhere to truthfulness..." And when he was asked: "Can a believer be a liar?" He said: "No." Second, you are instilling fear of devils in people, which leads to secret fear and belief in jinn and seeking refuge in them, and these are forbidden acts that can reach the level of polytheism (shirk). Allah Almighty says: {And indeed, there were men among mankind who sought refuge in men among the jinn, so they [the jinn] increased them in burden [of sin and fear]} (Al-Jinn 72:6). And Allah Almighty says: {It is only Shaytan who frightens [you with] his allies; so fear them not, but fear Me, if you are [truly] believers} (Al 'Imran 3:175). You must rectify what you have corrupted with your wife, so that her actions are out of fear of Allah, not out of fear of devils.