XmlHelper.cs

using System.IO;
using System.Linq;
using System.Xml.Linq;

namespace Helpers
{
    public class XmlHelper
    {
        public static string RemoveAllNamespaces(string xmlDocument)
        {
            XElement xmlDocumentWithoutNs = RemoveAllNamespaces(XElement.Parse(xmlDocument));

            return xmlDocumentWithoutNs.ToString();
        }

        public static string RemoveAllNamespaces(MemoryStream memoryStream)
        {
            XElement xmlDocumentWithoutNs = RemoveAllNamespaces(XElement.Load(memoryStream));

            return xmlDocumentWithoutNs.ToString();
        }

        static XElement RemoveAllNamespaces(XElement xmlDocument)
        {
            foreach (XElement XE in xmlDocument.DescendantsAndSelf())
            {
                // Stripping the namespace by setting the name of the element to it's localname only
                XE.Name = XE.Name.LocalName;
                // replacing all attributes with attributes that are not namespaces and their names are set to only the localname
                XE.ReplaceAttributes((from xattrib in XE.Attributes().Where(xa => !xa.IsNamespaceDeclaration) select new XAttribute(xattrib.Name.LocalName, xattrib.Value)));
            }

            return xmlDocument;
        }
    }
}