As long as you don't do anything punishable, their report will be false. And false reports get thrown out without any punishment being given to you.
On top of that, reports don't stack, even if they somehow trick everyone into reporting you it won't actually do anything more than if they reported you by themselves. Whether you would be punished or not will be based entirely on your behavior in that game, not on who reported you or how many of them there were.
They cannot get you banned just for playing poorly on a champ you don't know how to play. Riot can tell when you're having a bad game like that, and they know when you've done nothing wrong.
Have faith, don't react (just mute them), and report them after the match. As long as you do these things then nothing will happen to you, and they will face punishments for harassing you. If not immediately then eventually in the near future.