Restaurant refuses to refund us for over payment

We went to celebrate my mom's 50th birthday at Gold reef and the service was great until I realised that I had over paid them we had 18 adults, 4 non paying kids and 4 paying kids but instead I erroneously wrote 19 adults meaning we paid for a non paying kid. After we informed the staff no one is answering our calls for either the refund or a voucher for the the R210. Very disappointing service