Disable the Font Library

Removes the Font Library from the Site Editor, so fonts cannot be installed or uploaded from the editor.

Code

add_filter( 'block_editor_settings_all', function ( $settings ) {
    $settings['fontLibraryEnabled'] = false;

    return $settings;
} );

Fonts that are already installed keep working.

Install WPIDE on your WordPress site (it is free) and open Code Snippets.
Use snippet above adds it from the library in one click. Or copy the code into a new snippet with the same code type.
It is added inactive: review the code and settings, then switch it on.