Administrate your users and roles in the system yourself. You can create them in your own company or individually in your customers companies. For example, you can provide an external accountant or auditor who you work with, access to one or more of your customers. You can decide how much / how little they need to access.
When you create users and choose which things in the system they need to access, it is not only to "block" things. It is also about making the system simple for each user. If, for example, you do not have access to the documents, you do not see the menu for documents.