I believe the correct address for abuse reports is at the bottom of https://letsencrypt.org/repository/ (“Certificate Problem Reports”), however Let’s Encrypt may be reluctant to revoke this certificate - they only certify that your connection to given domain name is encrypted (a.k.a. DV - Domain Validation), not the identity of the person/organisation behind the website (or whether the website is safe to use).
See also blog entry https://letsencrypt.org/2015/10/29/phishing-and-malware.html and related discussion thread The CA's Role in Fighting Phishing and Malware.