If you have custom certificate templates that you no longer want to see, you can easily hide them by clicking Manage Certificates.
When in Activities Builder, make sure "Attach custom certificate of completion" is checked. Then click Manage Certificates
From there, you can simply activate or inactivate your certificate templates.
If you have a long list of certificates, you can use the "Search Templates" option at the top to filter down your results.
If you want to see the certificate in your list of current certificate templates, make sure it is checked as active, like this:
To hide, or inactivate, the certificate template, click the toggle.
Your changes will automatically save.
Once you've hidden the certificates you want, click close. The certificates that you've hidden will no longer display as an option next time you're viewing your active certificate templates.