I got recently harassed by someone streaming

Without too much context: If you were harassed during a LoL game in written chat ---> report at the end of the game like any other classic disruptive behavior from any lambda player. If you were orally targeted during a streaming or outside LoL ---> report on the adequate platform(s). You may provide more context here but no naming.

short answer: no. there are streamers like kaceytron who trolled for years with riot even saying "nothing we can do. what she does is the average of the elo she is in""

How can you be harassed in an online game? :O