Publish > Articles > Standard Item Edit > WYSIWYG

If you want to avoid Youtube videos from being automatically displayed on your articles then you will need to set the URL as a Link.

1.- Click on the Link option on the WYSIWYG field

2.- Insert the URL and leave the Text as the URL so it appears as a link (this is optional since you can also replace it with custom text)

The above will show the Youtube URL but the video will not be displayed.