This tutorial demonstrates how single node selection can be implemented. In this sample, parent node selection is disabled, and end-users are allowed to select only one leaf node within the ASPxTreeList control.
If you wish to have recursive selection enabled, it is necessary to additionally disable the ViewState by setting EnableViewState to False. You can find a sample in the ASPxTreeList - How to allow only one root node to be selected (Recursive = true) ticket.