Encontrado em Lifehacker :
When you play a song on Spotify, it uses some of your hard drive space to cache that file for faster playing later. Here's how to clear that cache if you need some extra disk space.
By default, Spotify limits its cache to 10% of your free space, which means the default setting should be okay—but if you've changed it, you could run into disk space problems. All you need to do is delete the files in the following folder (for your platform):
Windows:
C:\Users\USERNAME\AppData\Local\Spotify\Storage
OS X:
/Users/USERNAME/Library/Caches/com.spotify.client/Storage/
Linux:
~/.cache/spotify/Storage/