The way to curb this is to simply design the game in a way that, if the other player decides to just go around and kill other players all the time and looting their stuff, they should make it so they don't level up as much, and in some cases don't progress as much either.

The game allows for PVP, but it shouldn't be about PVP, yet the way it's presented has some players thinking it's PVP, so they'll act like dicks regardless, and thus that needs to be curbed right off the bat to deter that line of thought.

The other way they could curb PVP is them using GTA V's option to opt out of PVP, where your character becomes see through and cannot be killed/interacted with. At least then you can still play with friends, but won't be griefed by other players.