Manager accused me of *****- wouldn't use voucher

I am absolutely disgusted. My partner and I shop at Clicks regularly, and have never used any Cashbacks/Points or anything. In January, I received an awesome R18 off CashBack slip attached to the Clicks Newsletter. I cut it out and kept it safely in my wallet. Today, we visited your Longbeach Mall branch and when we got to the counter, I decided to use the R18 off slip. I produced it along with my Clicks card, and the cashier told me they don't use those anymore. She called the manager over who told me that they don't accept it anymore as I could have \printed it out on my computer"or \""already claimed it and now trying to claim more\"" She told me that my point balance on the screen is 0 and that the main member (my dad) could have used this voucher already and now I am trying to use it again. I was absolutely GOBSMACKED as now I was being accused of *****. I started getting quite emotional"