Block sensitive data fom dbo in SQL Server?

Like sa, dbo is the most powerful user in a database and no permissions can be denied to him.

dbo is a member of the db_owner database role, and these represent the equivalent of the sa/sysadmin pair at the database level.

Here's a query that can be used to find who is the owner of the current database:
select suser_sname(sid) from sys.database_principals where principal_id = user_id('dbo')

Hope this helps.