Here is a function that does this:

    public static void SaveAsUTF8WithoutByteOrderMark(string fileName)
    {
        SaveAsUTF8WithoutByteOrderMark(fileName, null);
    }

    public static void SaveAsUTF8WithoutByteOrderMark(string fileName, Encoding encoding)
    {
        if (fileName == null)
            throw new ArgumentNullException("fileName");

        if (encoding == null)
        {
            encoding = Encoding.Default;
        }

        File.WriteAllText(fileName, File.ReadAllText(fileName, encoding), new UTF8Encoding(false));
    }