Before I get started I would like to note this DOES NOT fully turn off the ads, they still play and still count but you cannot hear them. Another thing, sorry for linux and Mac users, I cant help you out but I'm sure you will be able to figure it out on your own using this guide.
First. Go to your sound mixer.
Second. Mute Steam Client Web Helper.
Third. That is it.
I will state this, when muting Steam Client Web Helper you do mute the trailers for games on steam but unmuting it before watching adds shouldn't be a problem. I have not found any other downsides to this than the ones I have already stated. Hope I helped