No one is immune. I've done it. What was the line in that Dilbert cartoon? Something like "You're pretty brave in cyberspace, flame boy."
A story to relate. I had a disagreement with a FReeper back in late 2000. Got some FreepMail that stated if the poster knew where I was, he'd promptly kick my ass.
Probably not the smartest thing to do, but I provided directions to where I work, how he could go to the Switchboard, ask for me, and we could sit down for coffee first, and if after that, if he was still intent on kicking my ass, we could go ahead and see where that takes us.
My rationale was that most Freepers are good folks at heart, and tend to get a bit puffed up behind the monitor. Never did hear from him, and noted a couple of years ago his account no longer existed.
Some other posters are correct - we sometimes have to just ignore some Freepers. There's nothing wrong with that.