I am 100% sure that this method had to work.
I used this script:
local StarterGui = game:GetService('StarterGui')
StarterGui:SetCoreGuiEnabled(Enum.CoreGuiType.Chat, false)
It is a LocalScript inside StarterGui. I want to disable chat because i use custom chat provided by Admin Script, but whenever i say something in chat, both custom bubble chat, both default bubble chat appears!

2 comments
Well, this is very odd, because my game is not FE, it is a LocalScript inside StarterGui, and i even tried to do this script without local part, by simply acessing StarterGui directly, and it still didn't work. It has always worked, and it is odd that it doesn't work now!
Try using the PlayerGui instead of StarterGui. You also might want to try using your script by starting the game, then reseting the player to see if it changes anything.