VERY bad service from Superbalist which is a first for me

I have always been very happy with the service and after sales service from Superbalist, RMA*** I ordered clothing earlier this year, and sent back some articles. According to Superbalist, one returned item was soiled and was not accepted by them. Therefore they promised to return it to me. This was confirmed on 5 May, and a promise was made that it would be sent to me. I have since then sent almost 10 emails, and received responses, all promising that it is on its way, with a waybill number. But nothing more than that, and the parcel cannot be trace. I feel totally helpless and frustrated and don't know how else to resolve this as emails get me nowhere: just more empty promises.