As to the OP's comments...
Have a look at example:
string xcontent = @"<Boards>
<Board class='support' appid='1'>
<Notes>
<Note class='message' id='1' source='client'>Good morning! My application hangs when i try to copy large portion of text to the clipboard.</Note>
<Note class='message' id='2' source='stuff'>Good morning!</Note>
<Note class='question' id='3' source='stuff'>Have you tried to copy less portion of text?</Note>
<Note class='message' id='4' source='client'>Wait a moment. I'll try.</Note>
<Note class='message' id='5' source='client'>Sorry, still the same error.</Note>
<Note class='message' id='6' source='stuff'>Thank you! Seems, it's a bug. I'll report it to the developer.</Note>
<Note class='question' id='7' source='stuff'>Do you have another question?</Note>
<Note class='message' id='8' source='client'>No. Thank you.</Note>
<Note class='message' id='9' source='stuff'>Thank you. Soon you'll get information about an update.</Note>
</Notes>
</Board>
</Boards>";
XDocument xdoc = XDocument.Parse(xcontent);
var nodesToClean = xdoc.Descendants("Note").Where(x=>x.Attribute("class").Value=="question");
foreach (XElement xele in nodesToClean)
{
xele.Value = string.Empty;
}
Result:
<Boards>
<Board class="support" appid="1">
<Notes>
<Note class="message" id="1" source="client">Good morning! My application hangs when i try to copy large portion of text to the clipboard.</Note>
<Note class="message" id="2" source="stuff">Good morning!</Note>
<Note class="question" id="3" source="stuff"></Note>
<Note class="message" id="4" source="client">Wait a moment. I'll try.</Note>
<Note class="message" id="5" source="client">Sorry, still the same error.</Note>
<Note class="message" id="6" source="stuff">Thank you! Seems, it's a bug. I'll report it to the developer.</Note>
<Note class="question" id="7" source="stuff"></Note>
<Note class="message" id="8" source="client">No. Thank you.</Note>
<Note class="message" id="9" source="stuff">Thank you. Soon you'll get information about an update.</Note>
</Notes>
</Board>
</Boards>