Are you...complaining that someone is plundering their own alt instead of you?
Only top 50 can plunder and on top of that it's an absurd amount of work to create a new account and level it up high enough to be in plunder range. Not to mention to be the first one to catch that SS when it comes up as an option before anyone else does. Then you only get one SS mission for free a day so unless you're spending ingots on *t account plundering yourself over some other player is hardly even worth the effort.
If you can plunder, attack them before they can attack themselves. Otherwise if they're wasting their plunders on their own accounts run a safe convoy.
Like if someone bothers to put that much effort into it, yeah it could be considered cheating and you can report it but also it doesn't even effect you so I'm not sure why you're the one that's so outraged?? Report and move on, it's hardly worth getting upset over...