From Xojo Documentation

Property (As Boolean )
aListBox.RequiresSelection = newBooleanValue
BooleanValue = aListBox.RequiresSelection

New in 2005r1

Supported for all project types and targets.

If True, users will not be able to deselect the last row by clicking below the last visible row or by dragging. You can still deselect the last row by setting the SelectedRowIndex property to ListBox.NoSelection.