Those pesky energy beings, always putting their noses into business of civilized lifeforms, like us! We'll make that pest pay. We'll just prepare a strong Faraday cage, lure the Poltergaist inside with Illusion powers, then clamp hard on its ass (if it has one) with several Teslas worth of magnetic field. Then we'll have an unorthodox power source we can sell to the daring, or keep it at our Hideout for $100k monthly income, amusement and occasional poking with electric prods.