Why not make a difference between locked and ban?

You can blame those who came before you for ruining that.

It use to be Riot would let chat restrictions infinitely stack, effectively providing the "locked" state you're asking for. However, those players kept using their limited chat inappropriately, and in some cases became the very trolls they despised.

After that we moved to the current system, which roughly translates to "don't be a jerk and you keep your account". It's pretty easy to not be a jerk.

What?

Those two types of punishments already exist. There are Chat Restrictions and bans for verbal misbehavior.

There already is.

Suspension and Termination.