Customer 62 (wasting my time)

I'm sitting here in sales and this customer comes in on our sales event day where everything is sales, commission and nothing else today. We are swamped with people who want deals and she comes to complain that she's not getting a discount credit for an offer she never was applicable for and keeps complaining about it every month (it's been 9 months already). So on this busy sales day I'm working with this long customer escalation on a non existent credit offer. Losing out on pay wasting my time. She bought 4 phones trying to use 2 buy one get one. However she was informed the offer is no longer valid as it has expired. And she keeps trying to get it!