I've never been attacked while umpiring but I have been threatened several times. The best thing to do to avoid this kind of thing is try and take care of situations brfore they escalate. Many inexperienced umps are afraid to toss players and coaches and things can quickly get out of hand. I'm not saying to be throw happy by if things are getting out of control get rid of the problem.
I have had fights among players which can be ugly. Most reconmend not to get into these and allow the managers stop it. I agree depending on the age. I had a fight between 14 yr olds in a soccer match a couple of years ago and was threatened with arrrest by the mother for breaking it up. I grabbed the aggresser around the chest from behind and pulled him out of it and she felt I choked him. I have worked security in many venues including concerts, NFL football and World Cup soccer and I know how to break up fights without hurting the compatants and certainly don't have to choke a 14 yr old to do it.