While Player Vs. Player interaction is a feature of EUO, once enabled by the Slash_Commands "/pvp", repeated and continuous killing of clearly outmatched players (aka newbies) is frowned upon. As such, 15 kills in a row while in criminal status will temporarily bar you from PvP mode. This lasts for 450 minutes (or 75 minutes if you meditate at the Shrine of Humility). See Shrines for location.
Note; Having resurrection sickness also disables PvP interaction as well.