Hello Vladimir,
There isn't a supported way to delete a custom Vendor Certification once imported to the system.
We document this here:
"Important! To avoid possible data loss, always back up the certification directory anytime you extend, create, or update a vendor certification. Metric families and vendor certifications cannot be deleted from your system. Test custom certifications before you implement the changes in your production environment. "
You can find it at this URL for the documentation:
https://docops.ca.com/ca-performance-management/3-6/en/building/self-certification/create-or-extend-vendor-certifications
Thanks,
Michael