What is meant with "editing" here?
The possibility to type text immediately into the control, like is done with a TEdit? Here the styles csDropDownList, csOwnerDrawFixed, csOwnerDrawVariable can be selected to prevent this. And in fact, the property "ReadOnly" switches to an unchangeable "true" state in this case.
Or the possibility to change the selection? Here, I can imagine only to prevent a change by code which memorizes the ItemIndex in the OnDropDown event and restores it in the OnChange event - there is a short flicker showing the changed ItemIndex, though.