Unfortunately there are a couple issues with this. While permabanning all trolls would is idealistic it is simply a HUGE task.
First you have to define trolls. Intentionally feeding is a type of trolling but is reportable.
After that there is a fine line between trolling and "having fun". For instance, I sometimes enjoy playing Thresh top lane. Is that trolling? I do pretty well with a good score and can do a decent carry job late game. All champions can theoretically jungle or do any role. Just because they aren't meta and offbeat doesn't mean its trolling; could just be fun.
Next, the feedback system for letting players know if a reported player has been disciplined is quite poor at the moment. This leads to dissatisfaction within the community because it feels like nothing is being done on Riots end. An improvement to this, which I THINK Riot is working on, would be a great help.
Finally, permabans don't do too much overall. They force a player to restart with a new account which does suck, but most players can avoid permabans for awhile after hitting level 30. I know there used to be a problem with account selling and that may also help permaban players immediately get back into the "endgame" of league with ranked and level 30 normals.