Trollers

Im sick of this trollers.I started ranked game everything was good.Someone flamed idk why and 1 guy said i will ban zed. I banned him before he did and said : "You can ban other i alredy banned zed".That "flamer" guy said i go troll you banned my champion and locks soraka top lane."Flamer" guy and jungle were premade.Jungler locks Ashe and said hahah noobs you will all lose -20 lp.I didnt want to lose 20 so i left matchmaking and lost 3 only.

Can someone help me how to prevent these trolls.I know this was my fault but this "flamer" guys was so upset that he wanted to troll. P.S-Flamer guy said he is diamond 1.....