Unfortunately there has never been an easy way to prevent related videos using the standard Divi video module. Somehow getting ?rel=0 into the url once worked, but no more.
As of Sep 2018 YouTube (aka Google) deprecated the ?rel=0 parameter, their revenue always being more important than user preferences.
There is a slightly ugly but possibly interesting work-around.
<iframe width="560" height="315" src="https://www.youtube.com/embed/heP9ddvgOz8?rel=0" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
While ?rel=0 still doesn’t prevent related videos, what it now does is only choose videos from the same YouTube channel. If you are able to set a channel per website this almost turns unwanted related videos into a useful ‘watch more’ option – as perhaps it was in ancient times.
If you’re not a regular user of YouTube – or are willing to maintain a channel exclusively for a website (by setting up a google account for it) – then you can just post the one video on that channel and no related videos will be shown.