How to report someone outside of the game?

Hello, This seems to happen quite often. Someone will add me after a game, call me a bunch of racial slurs and block me. I assume they do this so that riot doesn't see how they're racist and are trying to dodge penalties for being toxic. I managed to grab a screenshot of this last one, since its happening so often I was ready this time. What can be done? How can I report this? Thanks.
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