Reading between the lines (banks?) of the Cherwell, it seems that OUCS have placed a block on the music-sharing application, which was already banned under Oxford University network rules.
The reason is simple: OUCS houses a connection to JANET, which happens to be one of the faster Internet backbones of the country. Peer-to-peer applications are designed to use the bandwidth of their peers to spread the load away from the single server of the standard client/server model.... and that means that P2P software tends to saturate JANET as much as it can.
That's a bad thing. So it's sensible to prevent its use.
So, Oxford have not recently banned Spotify; rather, they've imposed technical restrictions preventing its use, to replace the logical ones that apparently people weren't following. It was banned all along, people! As was the original BBC iPlayer Downloader and Channel 4's equivalent, until they moved away from the peer-to-peer model. And in fact Skype was included in that list for a while, before a special relaxation was granted with certain configuration options.
But I guess they had to save a story for Fifth Week...