Why is self-defense a thing in a courtroom, anyway?
Simple: Because you don't have a choice about whether or not that guy's fist is going to hit your face. Specifically, there are no actions you can take, other than fighting back, that can prevent the imminent injury to yourself. Self-defense is valid in a courtroom because it is the only mechanism by which you can protect yourself from physical injury. The only options you have to defend yourself from harm involve inflicting harm upon another.
In League of legends, any chat messages you read, you read willfully and with full compliance. You are choosing to read them. You can choose not to read them, without having any affect on the person sending them. You can mute the person without having any effect on them. You can defend yourself from harm without inflicting harm upon another. There's no reason to engage the person as "self-defense" when you can achieve the same result with a unilateral action.
tl;dr You cannot mute a fist. You can mute chat.