This article explains the mouse support of the control.
RadComboBox responses to the mouse wheel scrolling. There are two possible cases:
When the dropdown of the RadComboBox is closed, then rotating the mouse wheel will change the selected item.
When the dropdown of the RadComboBox is opened, then rotating the mouse wheel will not change the highlighted item.
Note that in order for the RadComboBox to response to the mouse wheel scrolling, the mouse cursor should be over the combo box.
Disable MouseWheel Scrolling
In order to disable the mouse wheel scrolling, you should set the RadComboBox's IsMouseWheelEnabled property to False.
<telerik:RadComboBox x:Name="radComboBox" IsMouseWheelEnabled="False"/>