From the YouTube help:
You will automatically be opted into showing ads on embedded videos if you've associated your YouTube and AdSense accounts and have enabled your videos for embedding.
If you don't want to show overlay ads on your embedded videos, you can opt your videos out of showing overlay ads, though this will also disable overlay ads on your videos on YouTube.com. You may also disable your videos for embedding.
https://support.google.com/youtube/answer/132596?hl=en
Another technical solution could be to use a custom video player, and streamline the youtube video with that one. Have not tried but guess that the ads cannot be displayed in a custom player. However, could be forbidden.