I can’t tell you how to do it, but I can tell you how our group did it. In trying to get in with the Reconciled we discovered there was a play for the leadership of the faction, with an ancient ghost called Abraham calling the shots, and the freshly dead Rorik, now going by the name Ingram, making that play. Our job, on behalf of “Ingram” was to destroy Abraham, leaving the seat open for him. After that we’d have a powerful ally in the Reconciled.
So we did some digging and found Abraham’s real name, we staged the scene of his most glorious moment - setting alight sinners in one of the Six Towers parks near Rowan Manor - we used one of the Red Sashes and spiked the flames with electroplasm so they were puissant against spirits.
Then our Whisper compelled his spirit, using his true name, to possess the soon-to-be corpse and held it there while the body burned in the fire.
Okay, so that’s bragging. I think any involvement of Electroplasm would do the trick. If you tick Arcane implements you can have “a vial of electroplasm designed to shatter on impact” we all (in our game) imagine the stuff is charged and volatile and can harm spirits that it touches - hence electroplasmic ammo on the Hound sheet - ditto a Whisper could pull current from a nearby power cable to fry a ghost, or use Tempest.
What would the Spirit Wardens do about someone creating ghosts?
I imagine they would not be too happy. You should probably start a clock representing their investigation. They would track down and find some way to interrogate those ghosts.
They might lean on the Bluecoats to arrest or hassle the crew depending on the level of nuisance involved, I guess. I would drop their status with the Spirit Wardens slowly and use entanglements to portray the net closing in.
Like, the Bluecoats grab you in connection to the disappearance of one of your victims, your contacts and friends get hassled by agents of the church who show up on their doorsteps. Members of your crew who are devout followers of the Church maybe hear a sermon decrying the very thing that you’re doing and starts to speak up against you. A weird spectral force invades your lair, not a ghost, but clearly sent by someone to watch you.