I think forbidding PvP is kind of gay and breaks immersion. If you think the group for which you're running a game is made up of the sort of people that will be at each others' throats for petty grievances, always avoid any systems where rolling a new character will take up a lot of time (e.g.: Shadowrun), or make sure they have rolled 4-5 characters each before starting the campaign. I knew a group that always did the latter, because they had been playing together for so many years that they knew they'd start fights over minor disagreements and that usually ended with more than 2 dead PCs. For example, one such fight was during a D&D campaign over which inn the group would stay at. They started fighting in the street, and as the crowd gathered around the mage cast Confusion, which led to a massive brawl in the streets of the village. After the spell wore out, most of the group was lynched by the villagers.
Another one from the same group was when they had to implement a scorched earth strategy against an advancing orc army. One of the players had qualms about kicking an elderly couple of farmers off their farm and burning it all down, and another player started ranting about "let's make the beds for the orcs and tuck them in, and cook a nice stew - we can't let them go to bed hungry!". Anyway, fighting and hilarity ensued, and those two players communicated by proxy for months on end.
Back when I was a teen, we had a game of basic 1st edition D&D going on every Saturday, and the GM eventually got tired of PvP happening every other session, so he implemented a bare fist only rule for PvP, to prevent PC deaths, but it ended out being counterproductive because with the system he had put together, certain attacks were far more efficient than others regardless of armour class, and we exploited the shit out of that, PvP frequency increased, and some PCs spent half the campaign knocked out. That was fixed when we switched to Call of Cthulhu, where there's no fighting over loot.
Edit: another funny thing about the aforementioned D&D game is that we played it in a church's basement, which was used as a social centre for a bunch of cultural activities. My God, the amount of swearing and blasphemy that went on in those sessions: "I shit on God!", "I shit on the Virgin Mary!", "I shit on the Holy Wafer!", "I shit on your fucking mother!", and many such expressions common in Northern Spain. Thankfully, the parish priest seldom visited that part of the church and never got to hear any of it, because he would probably have expelled the roleplaying club from the premises.
