Why can't I change access rights on reports that are already created? Especially as an administrator?

You can.
Under the settings for a table, choose roles then a specific role. There you will see a hyperlink for "set report access

This only works for the reports showed in the reports panel, but it does not restrict access to this reports if those are already in a page the user have access.

I do not know how to deny  access to report base in user roles once they are already placed in accessible pages.

I do not know how to modify the access option in the reports from common to not common.

The "May be viewed by " option does not control access to the reports once they are in a page the user have access.