2019

First, you opted into private communications with this user. Just as your phone company does not police your text messages, Riot tends to not police private messages unless there is some sort of real life threats, or other extreme and potentially dangerous toxicity.

But regardless, the boards can't do anything for you - why don't you just contact Riot Support?

Don't accept friend requests from the person who flamed you last match. You are actually begging to be flamed at that point