Immediately bring in a demon when 5+ WP are spent, perhaps multiple if 7+ perhaps greater if 10. You might even keep count in the battle and that may have consequences.When the PCs go full nova and blow a lot of Willpower on activating their talents.
While there isn't a specific rule that allows for this there are hints that you can see in some of the magic item descriptions and demon abilities, even spells themselves. So using overpowered spells/talents/kin abilities have consequences.
If a permanent detect magic ritual were to be cast on a scrying device with a Hex or 7 it would give a ping if a large amount of magic was cast in a single area. This could be useful for a demon who feeds on that sort of energy or anyone that wants alerts about high power threats in the area.
The reason why this is devastating is they just blew a huge buffer of WP and now may be vulnerable. While it maybe is meta-gaming, I believe it is still fair considering the Forbidden Lands and the demons/sorcerers present.