Attacking in Pirates is limited to once every 5 minutes. Furthermore, They can only sink a ship if they have NO map, and a sail left. If they do not have a sail left, they can not attack in Caribbean. So there is some sort of limiting, just apparently not the kind you are looking for. I know it sucks for some of the smaller players, but that should be more incentive to grow strong so people can not sink you. You could also join one of the stronger guilds and sail with a strong player so you aren't sunk. Caribbean is meant to be competitive, and I doubt they will be changing that any time soon.