If there is a way to do that I would like to know because someone friended me after a game just to harass me after an admittedly bad game on my part. If someone from Riot or anyone who knows how to do that can tell me how if there is one it would be much appreciated.
You can always submit a support ticket with a screenshot.
Don't put yourself in that position, though =P Friending an abusive person post-game is never a good idea.
The best thing you can do is unfriend that person. That way they can't harass you any more. But yes, you can submit a ticket to Player Support and attach a screenshot of the harassing conversations. Support will most likely tell you to unfriend and/or block the person, and thank you for the report. They are not able to tell you if any action is taken against that player (due to privacy), but they will look into the matter and see if action needs to be taken.
