There is no site setting to remove the AI Assist menu item in the left menu. There are a few workarounds:

Element blocker for uBlock Origin

(probably also applicable for other adblockers)

Add the following custom rule to uBlock Origin under "My Filters" to remove the menu item:

###nav-askstack

Userstyle

Requires using an userstyle manager like Stylus:

Direct install

/* ==UserStyle==
@name         Hide AI Assist menu item in the left menu
@description  Hides the AI assist menu item on all SE sites
@namespace    https://github.com/PurpleMagick/
@version      1
@author       VLAZ
@license      MIT
==/UserStyle== */

@-moz-document domain("stackoverflow.com"), domain("superuser.com"), domain("serverfault.com"), domain("askubuntu.com"), domain("stackapps.com"), domain("mathoverflow.net"), domain("stackexchange.com") {
    #nav-askstack { display: none !important; }
}

Userscript

If you prefer using just a userscript manager like Tampermonkey or Violentmonkey:

Direct install

// ==UserScript==
// @name            Hide AI Assist menu item in the left menu
// @description     Hides the AI assist menu item on all SE sites
// @author          VLAZ
// @grant           GM_addStyle
// @grant           GM.addStyle
// @inject-into     page
// @match           https://stackoverflow.com/*
// @match           https://serverfault.com/*
// @match           https://superuser.com/*
// @match           https://*.stackexchange.com/*
// @match           https://askubuntu.com/*
// @match           https://stackapps.com/*
// @match           https://mathoverflow.net/*
// @match           https://pt.stackoverflow.com/*
// @match           https://ja.stackoverflow.com/*
// @match           https://ru.stackoverflow.com/*
// @match           https://es.stackoverflow.com/*
// @match           https://meta.stackoverflow.com/*
// @match           https://meta.serverfault.com/*
// @match           https://meta.superuser.com/*
// @match           https://meta.askubuntu.com/*
// @match           https://meta.mathoverflow.net/*
// @match           https://pt.meta.stackoverflow.com/*
// @match           https://ja.meta.stackoverflow.com/*
// @match           https://ru.meta.stackoverflow.com/*
// @match           https://es.meta.stackoverflow.com/*
// @match           https://stackoverflowteams.com/c/*
// @namespace       https://github.com/PurpleMagick/
// @run-at          document-end
// @version         1.0.0
// ==/UserScript==

(function() {
    const addStyle = typeof GM_addStyle !== "undefined"
        ? GM_addStyle
        : GM.addStyle.bind(GM);

    addStyle(`#nav-askstack { display: none !important; }`);
})();

Workaround - hide the left menu

Technically the only on-site option to do something about the menu item is to hide it.

1. Go to your profile preferences
2. Switch off "Show left navigation".

Then the left menu will be collapsed. It can still be accessed from the top-left next to the site logo if really needed :

I turned off the left menu years ago and find it is very rare that I even need it. It seems to house mostly marketing-related stuff with a few links you do not necessarily need.