How to disable related videos at the end of the embedded videos

Set rel parameter to 0 

https://www.youtube.com/embed/qU_djsolPkE?rel=0

In this case, rel=0 means that YouTube will not show related videos at the end of the embedded video. Setting rel=0 disables the display of related videos from other channels after the video playback is completed. If you were to set rel=1 instead, it would allow related videos to be shown.

This parameter is often used by website or content creators who want to prevent viewers from being distracted by other videos after watching the embedded video.

This parameter not working since 2018. See Notes below

YouTube Embedded Players and Player Parameters  |  YouTube IFrame Player API  |  Google for Developers