– Update: 26 december 2015 –

How to force Spotify to play your own local tracks from Spotify servers and not from your hard drive, so musicians will get paid, and you get the highest quality audio.
Local files show a rectangle icon with a music note (btw: here’s a description of Spotify icons.) In Spotify you see a list “Local files”. Select all tracks there, and hit delete. But this is not enough.

Start iTunes while holding Alt. Choose a new library, call this “iTunes empty”.
Spotify checks your iTunes library and imports your local files. It also checks regularly, let’s say every minute, if new local files are found in this folder.
In preferences, click Add Source for local files, add your empty folder and untick the rest.
Now start iTunes again while holding Alt, but this time choose you old (full) library.
From now on your songs don’t show up in Spotify anymore, so the music will play from their servers and musicians will get paid.

Another benefit: if your files were low quality, you now enjoy high quality / remastered audio files.

Oh, and one more thing: in the App Store is a free App called Reign Spotify. It’s a remote control, so you can control Spotify from any browser in your local network. Very nice.