Open iMovie and create a new project. Go to the "Media" tab and find the song that you want and then drag it to one of the projects available audio tracks.
You will need to wait a minute or so for it to import, this waiting is the longest part.
Then, grab any image and add it to the video track.
Almost done, now click on "Share" in the menu, and then "Share..."
A dialogue box will open, select the "Quicktime" Tab.
Under the "Compress Movie For" drop down menu, select "Expert Settings"
Now press "Share" and another dialogue will open. At the bottom, next to the word Export, select "Sound to AIFF"
Click "Save" and you are done. Just let it process for a minute or two.
You see, you need the random picture to make iMovie think that you are putting video or some kind of slideshow on top of the audio, but what they apparently don't realize is that you can select to export just the protected audio in AIFF format, which is an uncompressed format that cannot be protected.
You could also burn the track that you want to a CD, and then import it unprotected, but this is slightly faster, and it doesn't waste a blank CD.
Enjoy