Groupon

For 5 months I have been asking GROUPON for a refund on a voucher for a hair salon. For 5 months I have been sending regular emails and getting 'we are looking into it' responses. My complaint was that the hair salon wanted to charge an extra 20 euros for hair below the shoulders and this was never mentioned in the ad or voucher. Now GROUPON is saying that I used the voucher which is absolutely not true. Besides give up and warn everyone never to use GROUPON (which I will do), any recommendations on what else I can do?