If you're inherit your column from the DropDownListColumnBase or its inheritors you can set DropDownStyle property to DropDownStyle.DropDownList to disable typing in editor portion of dropdown.
If you're using your own implementation of ISupportsCustomDropDown interface you can add following code to disable typing in editor protion of dropdown:
[c#]
public override bool MakeEditorReadOnly(NodeBase node)
{
//returning true disables editing in inplace editor
return true;
}