I believe if your users can still see the button and actually click on it and create a new item, the modification you made may not be sufficient for your test users.
However, to answer your question, the only mean for you to hide something on the page (that I know of) would be a SPFx Application Customizer with global css in it. Please note that there will be a delay before such css is loaded (the delay before your JS is dynamically loaded in the page) and that the bar will still appear for the first instants.
You can load such CSS based on whatever condition you want (user not belonging to a group..) using JS and the SharePoint Rest API.
Don't hesitate if you need more info,
Hope this helps!