Why is that objectionable, do actual bad actors typically stop trying after being told to do so?

Which just demonstrates that these guys are actual bad actors, so blocking everyone at the university seems like a reasonable attempt at stopping them.

It's objectionable because of severe consequences beyond just annoying people. If there was a malicious purpose, not just research, you could bring criminal charges against them.

In typical grey hat research you get pre-approval from target company leadership (engineers don't know) to avoid charges once discovered.