if you don't like our coffee? why do you always buy it?

there's a woman who often comes to our store to buy a senior coffee. if its not a regular food service worker, she'll con them into believing our cappuccinos and hot chocolates are part of the senior coffee perk. no, you bought a senior coffee, not a senior cappuccino or a senior hot chocolate. well needless to say, she never likes our coffee, cappuccino or hot chocolate, and complains about it.
i caught her last time trying to get a cappuccino but did not say anything because she had gotten it while i was on break. the poor girl who dealt with her didn't realize that cappuccino did not count as a senior coffee, so when the woman complained she told her to wait for me to get back. i get back and my coworker explained to me the situation. i told her that the woman should not have gotten a cappuccino in the first place. i did not realize the woman was a constant offender.
there was nothing wrong with the cappuccino machine, she was just an idiot.
well anyways, yesterday i come back over to the food area while i was on my break to tell my coworker that i was going to ask our supervisor if she could stay over there longer so i could get things done. big mistake, i come over to find that same woman complaining about coffee again. well atleast she got a 'senior' coffee this time. as soon as she saw me she pointed at me and said "you usually make the coffee, why are there so many coffee grinds in the coffee? this is disgusting!" i actually did not make the coffee, new guy did and he probably didn't notice. she demanded i made new coffee. i told her i couldn't because i was not working. i told her if the grinds were an issue she could try filtering them out, she turned that idea down because i guess the coffee was 'too dark'. she kept insisting that i was the one to help her, i told her over and over again i couldn't cause i was not working and then left. i guess she ended up getting a food refund but it agravated me because she has done this multiple times. the icing on the cake was when she was telling other customers that they shouldn't buy our coffee as we were making a new batch.