First off WordPress already includes the ability to moderate comments. So unless you’ve disabled this in some way these comments can easily be moderated and deleted if needed.
I’m sure that with a bit of custom code you could prevent multiple comments; there may already be a plugin to do this for you.
But as an IT manager with over 15 years of experience under my belt, the idea of coding to handle what should be a management/HR issue throws up a big red flag to me. Technology is a wonderful tool, but in this case it feels as if you’re trying to use technology to address a one-off issue that will lead to a slippery slope of custom code and spurious requests to address issues that managers are unable or unwilling to resolve.
My suggestion would be to address the issue with the ‘trolls’ supervisor rather than attempt to customize the code.
I know this isn’t exactly the answer you were looking for but I do hope it helps.
Stephen