Hi,
I personally believe the mentioned website didn't breach Let's Encrypt's system, because the requirements for getting a Let's Encrypt certificate is just confirm domain ownership.
If you believe that website spreads mailware or other harmful information, you should report this directly to Google Safe Browsing and the domain's registry, since even Let's Encrypt stop issue certificate (if possible) to this site, there are still other CAs issuing certificate.
The above lines are my personal thought, and have no relation to Let's Encrypt or ISRG.
Thank you