I've honestly never tested this before, but it'd be worth testing if /mute <player name> works, even for people outside the game? It might. Beyond that, you can take screenshots (F12 while in game; they save to riot games/screenshots folder), and submit these to Riot in a support ticket. No guarantees they do anything extra about it (obviously you should block this person asap), but that's currently the only way to report these things.
That said, why did you even accept a request by 'some kid who was feeding' in your last game?