Classic Stream was available as an enterprise application that could be allowed or blocked. How can I block modern Stream?
A simple use case would be to block Stream until users have taken a training. Another one would be to block users from creating or editing content, or generating transcriptions, for compliance reasons.
Stream (on SharePoint) is just a better way to get to and play the video files already stores in SharePoint and OneDrive. There is no way to disable the ability to upload video files into M365 thus no way to disable Stream (on SharePoint).
However if you don't want videos to be able to be transcribed there is a SharePoint tenant setting and site setting via powershell to disable that.