Fairness in Punishment
#The 4-step method is close, but I'm simply suggesting they enact permanent ban to chatting IF/UNTIL that player starts to intentionally feed.
Intentional feeding should be permabanned regardless of whether or not you've been chat restricted or even ever used chat. Chat punishments should be not be connected to gameplay punishments. If it just so happens you've already been chat banned, and you start to ruin games by going Disco Nunu then you'll be punished for that accordingly--unrelated to the previous chat ban.
If they don't start intentionally feeding, then there's no reason to ban the account. They don't ever get to make friends or curse at people because of the chat ban, and that's the price they pay. --and Riot still gets their business.