@ser There is still the IP block option which really BLOCK's it, but is maybe also a little cumbersome.
You could look into using the package pfBlockerNG and then select one of two paths:
1: If you can force all clients to only use your pfSense as DNS you could block all DNS lookups that relates to Spotify. That would effectively either require a some good google-foo to find those names, or alternatively setup at test and have your DNS server log all queries when Spotify opens.
2: If Actual blocking is needed rather than just preventing nameresolution, then pfBlockerNG can also be configured to import lists that contains IP addresses. I'm sure there is some site somewhere that maintains Spotify's IP in a list - alternatively you could attempt to fetch the ASN ownership of IP blocks that Spotify owns, ,but that might not cut it (CDN's and such...)
Option 1 I ususally the easiest and best working model even though it only prevents nameresolution rather than actual blocking.