How do I report someone for racial slews after the game?

And that's why the league community is so terrible, riot lets them get away with it and does nothing about it. Did you know, you'd get more customers if you actually punished offensive people? This dude just told me to kill myself because I'm black in a non-reportable screen that took place on your client and through your servers and there is no way to put him through your system for that.

How do I report someone that split out rude slews after the game?