DMing to flame more and then blocking you to erase the evidence? How do I report this

Context doesnt matter much because Id like to know this for the future as well, but this is how it went down:

Usually I dont accept people that add you with the clear intent to talk even more sht over dm (when you mute people they feel the need to voice their anger if you didnt react to their verbal abuse). I saw the opportunity to screenshot the conversation and allowed him to dm me and do his flame shenanigans. Was my first time attempting this. But before I could save the conv he (apparently) blocked me, which closed down the chat and removed it from the client. Granted, since I cant add him back I cant open the conv.

Overall seems like a really weird implementation. Its a pretty save way for people to be toxic and not face the consequences. Maybe riot can still access these chat logs? If not, they ought to change something about this "loophole". Even if there were genuine blocks made, I see little reason to remove the chat for the one being blocked too.