I’ve been having trouble at work with the available bandwidth running out when I and my co-workers are using Spotify, a very nice piece of software, but just like Skype it tries to split its content through all available nodes and make sure they share it, which includes its users.
So what I did to force Spotify to only connect and fetch data from the Spotify servers was to use the built-in firewall in Windows XP and specify that
spotify.exe is only allowed to connect to:
18.104.22.168/22,22.214.171.124/21, mix up some traffic priority rules at the router for connections to those IP:s and Spotify can still be used, and not taking up all available bandwidth.
Updated 2011-08-23: Added a new IP-range that Spotify uses, 126.96.36.199/21.