Hey, I'm in a really sad mood but at least I'll try to help in any way I can, although I may mess it up. Alright, here it goes.
Yes, you have every right to be scared. I bet it feels like anticipating an attack at your house. It's very scary and it will mess up your nerves. Your stomach likely feels like it's full of butterflies. This is a rather serious issue so you have every right to feel the way you do. So what DO you do? There is some things you cannot control and it's up to the discretion of Nintendo to put this joker in his place and if it really bothers you so much, you can try playing private matches, playing in Octo Valley, or just playing something else until the issue resolves.
I feel the same way as you, I even prevented my own brother, who's level 44, to not play until the issue resolves itself. Right now, all we can do is wait for Nintendo to fix it. We can't stop him, we can't prevent him from screwing with other players, we have no power in regards to what he does. Only Nintendo can do something to stop him and you have to prepare for the outcome of them not doing anything. But you don't have to worry yourself sick when there's other things you can do to occupy your time.
You have a 3DS, right? You can play that. You don't need to play Splatoon all the time, and you may need to play something else, not just to avoid this joker, but for your own peace and mind. It'll likely get fixed within a couple of weeks so why don't you do the other things you love in the meantime? When you come back, the game will feel more fresh and more exciting the longer you were away from it. And you won't have to worry about some moron with the intelligence of a roach possibly ruining your experience.
