@Yvonne11 The terms of the Guest Refund Policy entitle guests to a refund at Airbnb's discretion if they choose to cancel their booking because an essential listed amenity isn't working. If they advance their case to Airbnb, you almost certainly lose the full value of the unused nights and also risk having a significant portion of the nights they stayed deducted from your payout. Unfortunately, the purchase of a new barbecue isn't likely to have any impact on the results, since it's an unrelated feature.
Your best bet would be to get in front of this by offering the guests a booking alteration to move forward the checkout date, refund the unused nights, and offer a partial refund (say, 20%) for the days that the oven wasn't working. These guests are clearly unhappy with their stay, and you have nothing to gain by prolonging it. In hindsight, when the ovens quit working, it would have been better to immediately give your guests the choice between relocating or accepting a countertop appliance such as a convection toaster oven as a substitute. You sadly lost the upper hand in the dynamic when the guests realized they could bully you into buying them a brand new grill, and this scenario typically ends in ill will and bad reviews.