Return the favor.
And no, not with being toxic. Instead, play obnoxious champions that tilt players just by seeing them.
Most people lose to a Yasuo or Zed or Riven (and many others) simply by seeing them selected in the screen. Malzahar is a good one for this too. Players view him as overpowered yet his stats are trash and he is much easier to punish than almost any champion out there.
So, just pick up some of the champions that make the game not fun for them or play strategies that are not fun.
I played against a Yasuo that had well over a million Mastery Points on Yasuo once and I actively refused to fight him at all (with the exception of the first time I killed him and when he dove me under turret). I would avoid trades at all costs and only auto him or hit him with abilities when it was completely safe, and even then almost always when he would keep turret diving me because he was cocky. I focused wave clearing and killed him every time he would dive me. He started to try to roam and I would ping my team and immediately shove the wave and take turret plates or do damage to turret until I took his turret.
/AllChat was filled with him complaining that I was boring and how my "strat was to push towers and not fight" and how stupid this was and so on and so forth. In the end of the game we won and he was utterly and totally irrelevant. I did not do this because I could not beat him. I solo killed him in lane for the first time just to show him that and then never fought him again. We actively avoided fighting him at all and pushed lanes, retreated and did not group except to grab objectives after getting a pick or something similar.
/AllChat was filled with streams of complaints to the point that I think he spent more time typing than he did playing.
Never once flamed him or even said anything except to his teammates (I congratulated one of them on a nice kill and had fun quoting my champion's quotes in /AllChat). At the end of the game I just commented that it is irrelevant how good you can fight if you are not allowed to. :)
All of this started because at the beginning of the game he said, "You must be Wood believing Malzahar counters Yasuo" (which he does).
The next thing he did was stand in mid lane just outside of my turret and spam "Back to Base" and then commented after my first kill on him, "Wait till I have items and 1v5 your trash team" and all kinds of random stuff about how I am playing a skilless champion (cept this was when Yasuo was blatantly OP and is far less interactive than Malzahar is).
Just change your tactics and make them regret it. :)