No user account should have unrestricted access to any other user account. The default permissions and ownership of user home folders should prevent that.
Not even an admin user has unrestricted access to other accounts. There's always an intermediate step that requires authentication, such as the authentication dialog in Finder, or the use of the 'sudo' command in Terminal.
If you're seeing unrestricted access by
any user over
any other user's home folder, then something is very wrong. How it got that way is unknown at this point.
First, you'll need to provide more details. Please post your OS version and computer model.
Second, you'll have to execute some Terminal commands. You can do this by copying and pasting what I post below, so there's no possibility for typos. You should also copy and paste the complete output in a reply you post here. Accuracy and completeness are required; guessing and assuming are bad.
Open Terminal.app, then copy and paste the following into a window:
Code:
id -p; mount; ls -la ~; ls -la /Users
Then copy and paste the complete output into a reply here.