In that case, use an e-mail based password reset, set a new password, and done, as far as having registered for the site, or contact the site's support.
This is bad advice. Do not interact with an unknown account opened with your email address.
A successful login from your IP address may be construed as assuming ownership of the account. They might try to collect money from you. Or, the account may have been used for illegal activities which are now linked to your IP address.
Never assume ownership of an unknown account. All communications (if any) with the account management should clearly state that you are not the account holder and are not responsible for the account. In particular, do not ask for the account to be closed. Asking the company to take action on the account may also be construed as assuming ownership of the account.
At best you can send an email stating you are not the account holder. Then put them on your block list. Do not get more involved than that.