To save bandwidth in a network or to enforce the company IT-policy you might need to block Spotify in some way. Spotify is using several ports to login(4070, 443 and 80). 443 and 80 is ports that you don't want to block.

Block the following IP-range: 78.31.8.0 MASK: 255.255.248.0
The user Daze have a nice solution for the Cisco guys at this forum but with 255.255.248.0 in the mask.

Then all you need to do is to wait for the first user to call and complain :)