1. Extract the widget templates from the Sitefinity SDK and locate the ContentBlockDesigner.ascx file (it's in the folder \SDK\Content\Resources\WidgetTemplates\Backend\GenericContent)
2. Copy this file to your website, such as into a folder named Templates.
3. Create a new entry in the viewmap with HostType: Telerik.Sitefinity.Modules.GenericContent.Web.UI.ContentBlockDesigner and Path: ~/Templates/ContentBlockDesigner.ascs (or whatever path is to your tempate copy)
4. Modify your copy of the template to hide this section.
5. Restart your website
I don't recommend you delete the section because it may be necessary for the element to be present on the page. Instead, wrap the section in a Placeholder and set the visibility to false so that it doesn't load on the frontend.