The possible way is to declare those fields as private or internal.
The alternative solution is to use DataContractJsonSerializer class. In this case you add DataContract attribute to your class. You can control the members you want to serialize with DataMember attribute - all members marked with it are serialized, and the others are not.
You should rewrite your ToJson method as follows:
public string ToJson()
{
DataContractJsonSerializer jsonSerializer =
new DataContractJsonSerializer(typeof(<your class name>));
MemoryStream ms = new MemoryStream();
jsonSerializer.WriteObject(ms, this);
string json = Encoding.Default.GetString(ms.ToArray());
ms.Dispose();
return json;
}