I had to do this years ago when someone registered my email address on an online dating service, I kept getting spam from it. I sent support requests, messages, help tickets, asking for them to remove my address or change it - no response.
Used the "forgot password" link, logged in, posted porn. Account deleted in about 5 minutes.
Did the site not have a way to delete the account once you logged in, or did you not feel like spending time looking for it given that you had already had your time wasted by them?
Used the "forgot password" link, logged in, posted porn. Account deleted in about 5 minutes.