brain dead customer services abound
I had a very similar exchange with Tesco customer services. I have firstname.lastname@example.org account and some idiot signed up to tesco clubcard using the wrong email address (mine). Tesco kept telling me they wouldn't reveal their customer's details, and I kept repeating I didn't want their customer's details, I just wanted them to stop using my email address. After much to-ing and fro-ing, someone at tescos must have actually activated part of their brain and did the necessary.
Other times I have had to do "forgot password" and then login to the offender's account and change their email address. I usually set it to "email@example.com" to match the domain for that service provider.