You can also report to Microsoft and Apple:
@kenorb, as other people have alluded to, Let's Encrypt is only trying to confirm that connections are cryptographically secure, and not that sites' content is not malicious.
If you want to pursue some kind of action against the site operator using the legal system, you might have the most success contacting the domain registrar. Let's Encrypt knows the IP address from which a certificate was requested and the contact e-mail address (if any) which was provided on the account, but will not divulge this information without a court order. Apologies for the misstatement; please see below. The domain registrar ordinarily knows these things as well as the physical address that the domain registrant claimed to be located at and the payment method that was used to pay for the domain name, which might be less anonymous than the IP address and e-mail address.
Some domain registrars may also be willing to suspend domain registrations for certain kinds of abuse complaints.