lets see...
the dumbest thing i've ever seen was done by me and my friends. We did an all yordle team with lee sin, and we spammed all chat with stupid yordle related things.
and our lee sin pretended to be a yordle all game which was hilarious.
I was practicing Thresh in a normal game with some friends, and I was matched against a Thresh main with Madlife in his name, much to my dismay. gonk
My Draven (who was D3 at the time I believe) and I managed to stomp bot lane together, then the enemy Katarina said "matchmaking at its finest. just push bot. all yours. worst draven NA tbh"
Their team then started calling me bad and made fun of me, even after I told them I don't play Thresh (to which they said "It's apparent" ).
Enemy Kha'Zix that tried to camp us but got ******** every time says, "Sorry you're not madlife"
I reply, "sorry your madlife lost wink"
Their team then got even SALTIER and spammed "yeah well you're reported/blocked thresh hope you get chatbanned buddy"
This was 2 months ago, but it's too hilarious to forget.
Another time, this enemy Akali in ARAM was more focused on complaining in all chat than playing the game because we had a Vel'Koz on our team. Stuff along the lines of, "gg fking riot ran out of champs to put in ARAM vel'koz bullshit champion on the other team 3 games in a row"