When the ad is playing you may see Sponsored X:XX and a little i in a circle in the lower left corner. Click the i. This will give you the ability to block or report the ad as appropriate. It will also show you what category has been associated to you as to why you received the ad, you can say if you want more or less ads of that category.

The iOS mobile app has a bug currently where you need to quit the app and come back after doing this, but it does block the ad and you won’t see that particular cut of the ad again. Unfortunately unless it’s a mid-video ad, or from a channel you’re subscribed to you will lose the video you’re watching as it hasn’t made it into your play history yet. But if the ad was mid-video your video can be resumed via history. For clarity, the video still exists, it’s just hit or miss if the video will be in the curated randomness they present you for you to try starting the video again.