How can I prevent a user to use admin rights to work around restrictions?
Well, somebody (aka at least one user account) needs to have full admin privileges on a Mac, there is no way around it. If you want to restrict the ability of an account, you will have to set up a dedicated user account (and define Parental Restrictions for it) and use this account for day to day work.

You would still need an admin account to install software etc, so you could for instance split the admin password in half, with each of you only knowing one part. Whenever an admin password is required, you both would need to sit in front of the Mac then and enter your respective half.