“Subject: Urgent Warning”

Honestly, I wouldn't even respond. I would imagine any response you give is going to be twisted as your original ones have been. You've tried to be reasonable, and you don't owe her anything - don't engage her any further.

If you desire to respond, I would relate the topic to her field. She claims to be a photographer. Surely she has a vague idea of copyright law. I would explain that you had developed a piece of Intellectual Property and licensed said property in a specific form -akin to taking a photograph. The person who takes a photograph owns the copyright. It just so happens that Spotify and Instagram, "enjoy your work" and have decided to make use of your work under the license you have given.

Definitely agree with one of the comments on the blog - the domain (haxx.se) really doesn't help the author's case here when trying to explain to a layperson.

Right - I know the person complaining is a little bit misguided (and seemingly refusing to understand, which can be frustrating) but when your primary contact is "@haxx.se" you must surely be at least a little bit prepared for these sort of things.

I'd prefer to give the author the benefit of the doubt, but it seems odd to even post this publicly at all unless a tiny part of them wanted to have a "haha look at this idiot" rant and they knew it could be justified under "I was only asking for help...".