Prisoner Island concept
So I did get why Riot doesn't want to make a prisoner island. Cause toxic player would not enjoy to play with other toxic players, so they would most likely stop after a while. Additionally they would see no hope at the end of the tunnel, cause they will see no positive behavior which would help them to reform. But what about a time-limit prisoner island? 1. chat restrictions, 2. than 14 days ban to think about it 3. and then 1 month prisoner island. If you're not happy after playing 1 month with people like yourself to play with nice people again, I don't know what else will help. 4. perma-ban (acutally I would prefer perma-prisoners island)
EDIT: What about now where Trump is president, we just make toxicity the average game tone. And people who haven't flamed or get reported for a few games will be send to prisoner island. I wouldn't mind to get some longer queues to play with nice people.