Not a thing you can do about the risk.
I was hit in the face once by a come-backer long ago. That shot ended my career as a SP pitcher. The pain & recovery from that broken bridge of the nose stuck with me for a long time. That was over 20 years ago. I still think of it today when I'm on the field.
I shudder today when, as a PU, I see F1s (or anybody, for that matter) get nailed anywhere on the body with a sharply batted ball. If I see F1 take a head shot, that's it ... dead ball, time out, and I have to place the runners after making sure the poor guy/gal can count my fingers. That's my responsibility as PU.
When I calmly think about it, I remember that come-backers are a risk in all softball/baseball variations. It's not just the head at risk, either. Injuries to rib cages, collarbones, and other body parts are always a risk, too. A simple face mask won't cure the problem.
Face it: if you're a pitcher, you assume the risk of a come-backer. You have to tell yourself you're fast enough -- either with your glove or to get out of the way -- to avoid harm at all times. And if you're not (as I wasn't), you'd best pick another position (I've since found out PU isn't the safest place on the field, either
).
In your "threat" situation: had I heard that threat, I probably would have used ASA 10.1.J-3 to eject him for USC. But it's a very rare batter who can actually carry that threat out. HTBT ... maybe he was kidding (and then again, maybe not) ... maybe the batter is just a jerk. At that level of play you never know; but it's best not to find out.
A ball field is no place for "blood revenge" play ... at least not one governed by ASA rules, and definitely not on
my field. I may be a rook, but I don't allow that c**p. Period.