Give them a chance in normals. If they continue to be toxic, remove them from the game or force them to do pve or custom only or something.
Yeah I get sick of these people too. Like a bot lane that goes combined 8/4/7 in laning phase, but the support says he's going to bed and keeps spamming surrender vote and calling the game hopeless.
Or the "this is my smurf account". "I'm really plat 4, and I can't carry you turd maggots". (as they lose lane while you are being camped by the enemy jungler and roamed on by the mid they fed, yet you have a better kda than them and more farm).
Or the, person who has to leave 20 min into game, so you have to win "like right now" or else you will be 4v5.
Or the person who dies 2 times in lane, decides game is over, sits in base pouting for 2-25 min because you didn't gank for them, then randomly comes back and plays.
Or follows you around in jungle just stealing your camps because they died 3 times by the time you cleared your jungle and ganked another lane for a kill. And you didn't gank for them.
Or...
you get the idea.