Use an offensive (but not overly so) in-game name (I prefer to have a name with a political side to it), don't chat with anyone and take everything they throw at you for using that in-game name. Then report them for verbal abuse and silently leave. People who flame for that are usually toxic players either "in recovery" or "in disguise".
[An example of what I did]
On a Turkish Server I used an inappropriate name, after about 3-4 months Riot changed the name for me. The name I used was "PKK Kurdistan". I played Zilean support 24/7 since in blind pick, support was the role that was usually left and Zilean had a PKK ring to it.
Excerpt from wikipedia on PKK:
"The PKK is listed as a terrorist organization by several states and organizations, including the North Atlantic Treaty Organization (NATO), the United States, the United Kingdom, Japan, and the European Union all of which are Turkish allies. However, the United Nations and countries such as Switzerland, China, India, Russia and Egypt, all of which are not allies with Turkey, have not designated the PKK as a terrorist organization."
I live in a country that does NOT deem PKK to be a terrorist organization. I understand why the name got changed (it was political and inappropriate for the region) and wouldn't have it any other way.
I began to notice that I am getting flamed for using the name "PKK Kurdistan" every single game. I don't speak Turkish so wouldn't talk back at all. After the game, I would report the abuser and leave. It is quite easy to understand if someone is flaming you as the insults (no matter what language) are usually the same. Google translate also helps.
Needless to say, every second game (if not every single game) I would get an instant report feedback message (I must have had about 50-80 of them in a span of 3-4 months) thanking me for improving League experience for the entire community.
P.S. I guess all the downvotes are because people of this forum do not like the tactic used, however, my point was not to present a neat/soft/cushy option. What I wrote above does exactly what it sets out to do with limited consequences for the person doing it.