Hi there emil-boctor-mark,
It's vital that only authorized employees have access to your books. QuickBooks Online is a great tool able to help you manage your books from anywhere with an internet connection. I can point you in the right direction to remove a user.
In this article, there's a section that shows you how to remove a user from your QuickBooks account. This will prevent them from having access to your books as soon as they are removed as a user.
If you have any other questions, feel free to reach out here.