Hey, just wanted to hop in here because some of the comments are pretty upsetting. The best thing you can do here, as someone else said, is to file a ticket through the Support website. There should be a place where you can upload screenshots and reporting at the end of game shouldn’t prevent you from doing so. If you are having trouble doing that, please PM me and I’ll help walk you through it.
My Reddit name is the same as my in game name so I encounter this kind of behavior pretty often (was... not worse, but different, before I also had Riot in my name). You are absolutely within your rights to report this person and also to be upset by this. No one should have to go through this and just resign themselves to it because “that’s how games are,” in League or any other game. This kind of stuff feels awful and “ignore it” and “move on” are not helpful advice here. I hear you.