When working on a shared network with limited bandwidth, it’s sometimes nice to be able to keep listening to Spotify without ruining your co-workers’ Internet connections.
Spotify is P2P (the desktop app is, anyway), so you both receive data from other Spotify users and transmit it to them. Blocking the outgoing traffic entirely would be un-neighbourly (probably also a violation of TOS), and might even prevent streaming from working altogether.
Fortunately, as I just discovered, ipfw allows you to add a pipe to a range of ports, all of which you can then throttle to a certain data transfer rate. It’s a shotgun approach, and will slow down any other services that are trying to send data over those ports as well, but since Spotify seems to stick to the range 10000 to 80000, and I rarely if ever run anything of consequence on those ports, this approach works for me.
sudo ipfw add pipe 1 ip from any to any out dst-port 10000-80000 sudo ipfw pipe 1 config bw 8KBytes/s