How could I disable the selection from the first item(root node) in menu ?
No items are initially selected in the tree. If you mean opened state of the root node then with free version you'll need to make changes to the
script and with pro version you can use 'st' key in item scope settings to specify initial state or use open IP to close the node.
There is a reason why root item is initially opened. Normally hierarchies have single root (homepage of the site) and when visotor opens your site the home page is opened. Then when visitor wants to navigate elsewhere he/she will need to open root item in order to see other options. We eliminate this extra actions by making the tree open root items initially.