How can I make the xmlserializer only serialize plain xml?

You can use XmlWriterSettings and set the property OmitXmlDeclaration to true as described in the msdn. Then use the XmlSerializer.Serialize(xmlWriter, objectToSerialize) as described here.

To put this all together - this works perfectly for me:

    // To Clean XML
    public string SerializeToString<T>(T value)
    {
        var emptyNamespaces = new XmlSerializerNamespaces(new[] { XmlQualifiedName.Empty });
        var serializer = new XmlSerializer(value.GetType());
        var settings = new XmlWriterSettings();
        settings.Indent = true;
        settings.OmitXmlDeclaration = true;

        using (var stream = new StringWriter())
        using (var writer = XmlWriter.Create(stream, settings))
        {
            serializer.Serialize(writer, value, emptyNamespaces);
            return stream.ToString();
        }
    }

This will write the XML to a file instead of a string. Object ticket is the object that I am serializing.

Namespaces used:

using System.Xml;
using System.Xml.Serialization;

Code:

XmlSerializerNamespaces emptyNamespaces = new XmlSerializerNamespaces(new[] { XmlQualifiedName.Empty });

XmlSerializer serializer = new XmlSerializer(typeof(ticket));

XmlWriterSettings settings = new XmlWriterSettings
{
    Indent = true,
    OmitXmlDeclaration = true
};

using (XmlWriter xmlWriter = XmlWriter.Create(fullPathFileName, settings))
{
    serializer.Serialize(xmlWriter, ticket, emptyNamespaces); 
}

Use the XmlSerializer.Serialize method overload where you can specify custom namespaces and pass there this.

var emptyNs = new XmlSerializerNamespaces(new[] { XmlQualifiedName.Empty });
serializer.Serialize(xmlWriter, objectToSerialze, emptyNs);

passing null or empty array won't do the trick