How to block a developer certificate
I’m not sure you can block it other than uninstalling it and whatever you did to install it (other installer, other app, ???)

To get Apple’s attention, unless they sold that app on the App Store or you were sure you can locate an app that’s signed from the Mac App Store or iOS App Store from the same developer, I’d start with Apple Support to report potential loss of that developer’s Apple ID.


https://getsupport.apple.com/


If you assume no developer would sign code that does bad things, you can also assume that somehow they lost their keys and someone is abusing their keys to spread bad software. If it turns out they are the bad person, you’re not making the assumption of anything other than their keys were used for something they may or may not have approved.