When an admin disables an equipment there would be a popup that asks if the system should send out an automatic email, notifying the users who booked that equipment, that it was disabled.
It would also have a time setting so admin could select how long in time it should notify the people (ex: every booking of that day, whole week, the next two days, etc).
That would allow users to not come to a disabled equipment because admin forgot to send an email manually 