Please refer to the attached gif file and the provided sample project. 

//Workaround 1          this.commandBarDropDownList1.CommandBarDropDownListElement.EditableElement.TextBox.TextBoxItem.UseDefaultDisabledPaint = false;

            this.commandBarDropDownList1.CommandBarDropDownListElement.EditableElement.TextBox.TextBoxItem.Enabled = false;

//Workaround 2

            this.commandBarDropDownList1.CommandBarDropDownListElement.EditableElement.TextBox.TextBoxItem.ReadOnly = true;
            this.commandBarDropDownList1.CommandBarDropDownListElement.EditableElement.TextBox.TextBoxItem.PropertyChanged += TextBoxItem_PropertyChanged;
            this.commandBarDropDownList1.SelectedIndexChanged += CommandBarDropDownList1_SelectedIndexChanged;

        private void CommandBarDropDownList1_SelectedIndexChanged(object sender, Telerik.WinControls.UI.Data.PositionChangedEventArgs e)
        {
            this.commandBarDropDownList1.CommandBarDropDownListElement.EditableElement.TextBox.TextBoxItem.SelectionLength = 0;
            this.commandBarDropDownList1.CommandBarDropDownListElement.EditableElement.TextBox.TextBoxItem.SelectionStart = 0;
        }

        private void TextBoxItem_PropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            this.commandBarDropDownList1.CommandBarDropDownListElement.EditableElement.TextBox.TextBoxItem.SelectionLength = 0;
            this.commandBarDropDownList1.CommandBarDropDownListElement.EditableElement.TextBox.TextBoxItem.SelectionStart = 0;
        }