see below code sample. I assume that you are familiar with the DiamondManager web service. if this is not the case, I would take a look here first http://technet.rapaport.com/Info/LotUpload/SampleCode/webService_Example.aspx.
DiamondManager _manag = new DiamondManager();
DeleteLotParameters dl = new DeleteLotParameters();
dl.ByField = ByFieldTypes.RapnetLotID;
dl.FieldValueList = "DiamondID_1,DiamondID_2.....,DiamondID_10";