Reading through the Malleus section of DotDG, this idea struck me like a slap in the face.
Why don't Radical Inquisitors of Ordo Malleus deal with these cults (obviously not including those that worship the Ruinous Powers directly) by binding the Daemonic patrons into Daemonhosts. The cult's own patron could then be used to destroy the cult, not to mention give information on cult activiities etc.
Obviously, this would be far from easy for the Warp sorcery using Inquisitor. Researching information such as the Daemon's True Name and acquiring the knowledge to conduct the ritual would be difficult but surely not beyond the means of said Inquisitor. Barring execution for heresy and given rejuvenat treatments, time would not be a constraint, the Inquisitor could spend decades, perhaps centuries on this research.
What would happen to the cult if this occurred? Or to individuals who had made Dark Pacts with the Daemon? Would the boons from the Pacts simple cease to exist? Would the cult immediately implode through sheer terror? The mind boggles!
For an interesting variant, the aspiring Radical (extremely Radical) Inquisitor could bind the Daemonic Patron into a weapon creating a Daemon Weapon. Fun and frolics ensue.
Why haven't any Inquisitors even thought of this? Or maybe they have....?