Add some environmental hazards and AoE attacks. Make the fires spread out of control and become a threat of their own. Tempt them with explosive barrels in dangerous places. Familiars die easily, but are cheap to resummon.
Keep attacking them frequently between rests. Make them reconsider that 1 hour familiar ritual and invisibility spell slot.
If even a single witness escapes, he's telling everyone what happened. Most spell casters can immediately put out the fire with Prestidigitation, Control Flames, or Druidcraft.
When they destroy items with fire, describe expensive things melting into worthless things.
Does that mean they took Pact of the Chain? If so, then it's a class feature that's supposed to be powerful. Maybe this signature trick makes their patron impressed, jealous, or bored...
They passed up on Pact of the Blade, so when they do get attacked, they're more vulnerable.