You can deactivate your account by using the link provided below.
However, our user Leslie (hi!) points out, your account won’t be deleted, it will only be deactivated. This means that your personal data will probably be stuck on Polyvore servers forever. For more info, see https://polyvore.zendesk.com/entries/21019103-How-do-I-reactivate-my-account-
You could try to contact them via https://polyvore.zendesk.com/entries/20966833-Contact-Us and ask them if they can delete your account (tell ’em we sent you!), but it remains unclear what happens to your personal data. Thus, blacklisted.