Had a friend break one of my crisis suits once.
The little metal part of his tape measure shot off when he retracted it once, taking off the arm of one of my crisis suits in my dead pile.
We couldn't find the arm but I thought it was the funniest damn thing I had seen in a game.
Complete accident. Odds of it happening again are fairly low.
Another time, someone was looking at my Powerfist Kroot (with my permission to touch it. I don't like people touch my models, but if you ask permission, I'll usually let you.)
He dropped it, the "powerfist" broke off. He apologized. I glued the arm back on and shrugged it off with an "accidents happen"
If a model breaks by accident, nothing you can do but Shrug it off and fix it.
If someone just breaks a model of mine on purpose, he is not allowed to touch them anymore.
Nice and simple. No hammers, no fighting. Don't let it get to you. Some people are donkey-caves just trying to get you to fight with them or just trying to get a rise out of you. Best to deal with them with a big smile and a "haters gonna Hate".
If someone starts going through all of your models one by one and breaking them and you are at a
FLGS, DO NOT get physical. Get a manager. The ass will be banned(and possibly arrested, like in deadshot's story) and you will be compensated. You get physical then you can get in trouble.
TL;DR
Leave the fighting to the Models.