I turned to Google Apps Script for a solution because I had a nice experience with it in the past. I was able to build a script within a very short time that does exactly what I need, but has some drawbacks:
- does not work for files which are owned (created) by the user you want to remove
- you need to manually run the script a lot of times, because there is a limit for how long a script can run continuously. Since Apps Script operates quite slow, you easily reach that limit. However, the script goes on where it stopped the next time you run it!
One could further improve this with a simple UI and triggers to re-run the script automatically before it reaches the time limit, but it did the trick for me so I thought I'd share it here!