Thanks for your answer @CodeWasher

I can't find where i can add custom CSS. When i click on "edit" i have a side menu with 3 parts "Blocks" "Customize" and "Theme". In Theme at the bottom in "Advanced" i saw i can make "Code Injection" but it's only for HTML, not CSS. 

I am sorry i can't put a screenshot because i dont have enough "karma"
Thanks in advance

Go to Website Editor:

Navigate to your website and enable the website editor.

Add Custom CSS:

Add the following CSS code to your website’s CSS:

.o_website_login, .o_sign_in {

    display: none !important;

}

Save the changes and publish your website.