b) absolutely - most people either don't use ad blockers, and of those that do, most people are not militantly committed to them. There will be a set of people who stop using the service out of principle, but almost all of them weren't making any revenue for spotify anyway.
The only risk to spotify I can see is if they use somekind of nefarious method to detect ad-blockers (eg root kits) and I 1) don't think they're that stupid 2) don't think they would be necessary or useful anyway.
I guess another risk is if they force people to turn off ad blockers, then accidentally serve them malware, but afaik, various other sites have done this and nobody seems to care.
Conversely, any (non-critical) service has the right to stop showing us ads or serving us in any other way.
The closer you are to a monopoly, the more required you should be to serve all. We don't allow the electric company to kill your power because you made disparaging emails or bad messages online.
And if you're a monopoly, you should be dealt with heavy handedly. Because power and money only further power and money. Anti-competition laws stem that to an extent.
Music streaming isn’t the kind of good that falls under common carrier so I don’t think the analogy of the electric company fits. People can easily go through life not subscribed to one. I for one only have iTunes Match which isn’t the all-you-can-eat style of Spotify/Apple Music/Play Music/Amazon Music/Tidal/etc.
Companies have a habit of trying to squeeze all kinds of stuff into their ToS/AUP, hoping they can get away with it.
Who'd want to bet Spotify's bans would survive both a) the courts, and b) public opinion? Remember they'd need to win in both.