This article explains how to remove manufacturers and their catalogues via Notify.

Removing Manufacturers

For easier visibility, you can group the Catalogues by installed Catalogues.

Click on the View tab at the top of Notify and select Group by Installed.

Once grouped, you will notice a green checkmark for the Catalogues that are installed.

To remove them, simply click on the one you wish to remove, and you will get a prompt.

Do note you will have to close CET before doing so. If you have the CIL for AutoCAD Add On installed, then you will have to close AutoCAD as well.