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?
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.