I was thinking that it is a simple task "select nodes using XPath expression and then to delete them from XML Document", but after I spent a couple of hours ... My fault was I searched for something likes 'Remove current node', 'Delete active node', 'Detach node from tree', etc.
Way to delete node is removing of it from its parent node:
XmlNodeList nodes = xmlDoc.SelectNodes("//node");
foreach (XmlNode node in nodes)
The Meat of the tip is code line
node.ParentNode.RemoveChild(node) which does removing XML node.