How do I remove purchased GarageBand content?
You should look for it in the following place, copy following and paste it in to the Go to Folder in the Finder.

/Library/Audio/Apple Loops/Apple/Apple Loops for GarageBand


Know-how:

*If you are wondering how I did it, since I do not use the Garageband:

I used the mdfind -name Garageband in Terminal that showed me all locations for it. After that it was easy.*