There's nothing built into PowerPoint for the web to make this possible. Viewers need your permission to view the presentation, so that lessens the chance that a recipient would be a software pirate.
As a workaround, create a low-res transparent PNG file, then place it over top of the slide with the video using PowerPoint on your desktop. If you have Microsoft 365, use the Selection Pane to lock the PNG. Then upload it to OneDrive/SharePoint and share the file. The video can't be right-clicked.