Right now you are grabbing the entire innerHTML of the div with the class postbody. It sounds like you want to grab the innerHTML of the first child inside the div with the class postbody.
Try something like this:
Code:
Private Sub Main()
Dim divPostBody = GetElementsByClassName(webbrowser1.Document, "postbody")
o = divPostBody.Count()
For Each div In divPostBody
Dim firstDiv = div.FirstChild()
topicextract.Text = firstDiv.InnerHtml
Next
End Sub
Private Function GetElementsByClassName(document As HtmlDocument, className As String) As IEnumerable(Of HtmlElement)
Dim elements = New List(Of HtmlElement)()
For Each element As HtmlElement In document.All
If element.GetAttribute("className").Split(" ").Contains(className) Then
elements.Add(element)
End If
Next
Return elements
End Function