Although there is no "enabled" property, you can simulate it by graying out the node and avoiding that anyone can click on it. Maybe this could work:
private void treeView1_BeforeSelect(object sender, TreeViewCancelEventArgs e)
{
if (Color.Gray == e.Node.ForeColor)
e.Cancel = true;
}
private void Form1_Load(object sender, EventArgs e)
{
foreach (TreeNode node in treeView1.Nodes)
if (node.Text == "sample")
node.ForeColor = Color.Gray;
}