Someone flamed me in personal chat on my friends list, how do I report this?

I had a bad game as thresh, I wasn't landing hooks because of the enemy team's mobility/dashes/speed ups, and was forced to not play Sona because my adc said she would feed if I did so, I picked Thresh because if I can't play my main I'm just gonna play something for fun instead. Well we did okay early on (I honestly hate low elo caitlyn players, they don't understand her kit or her play style of poking and zoning in lane and for some reason they always have bad attitudes and blame supports) so I did kind of start fooling around when I could but when a real fight was going on I was actually doing productive things for the team and at least TRYING to be a good player. So the game ends and everyone's like "garbage thresh" "R**arded thresh" and so on, then I get a friend invite from someone who literally only added me to curse me out and tell me I'm garbage and should get aids. I took a screenshot of this but where do I submit it to get this person banned or something because it's totally unnecessary and there's no out-of-game system for doing so?