Treeview Mvvm root node disable

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WPF

MVVM

I have a treeview writing with MVVM pattern that was done by other person. In the treeview, the root is UserGroupCollection and the child hierarchy is UserCollection. When the root is selected, Group Detail View will be shown in the side content. When child node is selected, the User Detail View will be shown in the side content.
There is content at the right side to show the detail view according to treeview items selected. Currently, both root and child nodes can be selectable and The side content changed according to root and child nodes selected. Now I need to make some changes. I just want to show User Detail View when child node User is selected. I don't want to do anything when root node is selected. I mean want to set disable or false to Treeview root node selected property. I just new working on MVVM and can't find the way how to do. I am expecting any suggestion and very appreciate your answer.