With PvPToggle Mod 1.7.10, 1.7.2 you can have players decide for themselves! Let players opt-out of PVP by typing /pvp off. Or, if they rather have their heads smashed in, have them type /pvp on

This mod has 4 config options:

• keepInventoryOnPVPDeath (default=false) Whether or not the players keep their inventory when they die because of a player
• keepExperienceOnPVPDeath (default=false) Whether or not the players keep their experience when they die because of a player
• showMessageOnLogin (default=true) Whether or not to show the player a message that pvpToggle is enabled on your server, and what his/her state of pvp is (disabled or enabled)
• coolDownInSeconds (default=30) How long a player has to wait before toggling his state of PvP. Thus counteracting the hit-and-run phenomenon. (set to 0 to disable)