Since they use VoIP SMS and "first name" it's pretty hard to resort to the old fashioned doxxing despite living in the golden age of data leaks.

I would suggest simply wasting their time as much as possible (I've lead on such people pretending to be naive and caused great frustration). But ideally, not to waste your own time.

I think in the near future android-local chatbots will be further along, or iphone-local, and ideally one can run the chatbot. and you can just set the chatbot to "waste this person's time" as we will have chatbots for "flirt with this potential date and schedule drinks". Of course the endpoint of such a world is chatbots all around and no humans.