Use a modern third party software called Deceive to do the job, it automatically blocks the connection to LoL Client chat servers, so nobody can see you even log into the account or play from it unless they stalk your match history or use third-party software like op.gg to check if you are in-game.
It is completely safe, it doesn't steal your data and doesn't even know it, it just blocks chat IP addresses from your connection
All you have to do is
Download the latest Deceive release
Launch League of Legends using the Deceive client with Admin mode instead of the standard LoL client
Very useful if you don't want to be bothered by friends or someone.