To report private key compromise, certificate misuse, or other types of fraud, compromise, misuse, inappropriate conduct, or any other matter related to certificates, please email cert-prob-reports@letsencrypt.org.
By the way, to revoke a certificate with an independent account, is it enough to prove control of one domain in the certificate, or proof of control is required for all domain names in the certificate?
Yes, you need to prove control over all the domain names in a certificate first before you can revoke it using a different account. See the "Using a different authorized account" section of the Revoking Certificates documentation.
Also note that revoking doesn't actually do that much; if the attackers can no longer route traffic for your domain to themselves then there isn't much they can do with the certificate, revoked or not. And if they can, then even if the certificate is revoked not all users' systems will check it.
Yes, I completely agree with you. We have regained control of our DNS and our domains along with it but we still wanted to clean up those illegitimate records.