Assuming WinForms, try canceling the edit in the BeforeLabelEdit
event:
public Form1() {
InitializeComponent();
treeView1.BeforeLabelEdit += treeView1_BeforeLabelEdit;
}
void treeView1_BeforeLabelEdit(object sender, NodeLabelEditEventArgs e) {
if (e.Node.Level == 0) {
e.CancelEdit = true;
}
}