It has nothing to do with whatever the user does to avoid being exposed to those ads.
This seems more akin to cable services here. For all intents and purposes, all of the channels they provide are served to the home, but they then restrict based off the package you’re paying for. In Spotify’s case instead of restricting (unless they have premium only channels/content), they’re adding advertisements. To block the ads is analogous to using a method to remove the restriction on channels you can access, and the typical remedy is to terminate the service to that customer and/or seeking civil remedies.
An analogy to cable services would be if you had something installed on your TV that automatically switched channel or put on a cat youtube video, when the commercials are on on and switched back when the commercials were done.
Your example would be an analogy to having a script on Spotify which gave you access to some premium music library that only paying customers should have access to.
If it isn’t against the agreement directly, it certainly is against the spirit of it. And that holds a lot of weight. Because what is the point of being able to show you advertisements if you’re going to block them from even rendering?
I get it. I don’t like ads either. Well, I’m generally fine if they make some degree of sense (none of the odd ones where it has a picture of an onion partially covered by a sock or something) or don’t hijack the browser (massive redirect that eats back history and presents me with some “congratulations, click here” thing). But if I don’t like the ads, I’m free to either pay for Spotify without advertisements or not use Spotify at all. No one is holding a gun to my head that I must have them (and I don’t after a promo period with Sprint) and they do have to pay licensing for that music (to include some kind of fee per x streams of given songs to ASCAP or whatever). That doesn’t even factor in the infrastructure requirements on their end. So why would anyone expect this to be totally free to a user?
8 Rights you grant us
In consideration for the rights granted to you under the Agreements, you grant us the right (1) to allow the Spotify Service to use the processor, bandwidth, and storage hardware on your Device in order to facilitate the operation of the Service, (2) to provide advertising and other information to you, and (3) to allow our business partners to do the same. In any part of the Spotify Service, the Content you access, including its selection and placement, may be influenced by commercial considerations, including Spotify’s agreements with third parties. Some Content licensed by, provided to, created by or otherwise made available by Spotify (e.g. podcasts) may contain advertising as part of the Content. The Spotify Service makes such Content available to you unmodified.