When you put it that way, the answer is obvious:

Assume VCs are brainless profit maximizers who don't understand ethics. How do you get them to say "I'm gonna stop you right there"?

Answer: Make it unprofitable to collect this data. Change the incentives.

So really, the correct answer IS on the legal level. Make a set of laws which make it burdensome at best and completely unprofitable at worst, and then the incentives within the system aligns.