How To Prevent DBA From Getting Confidential Data Stored In SQL Server?

Suppose I have a database storing some confidential information, such
as legal information, medical or financial records,  etc., and a
Web site with a membership system so that only authorized users can
view the information.



I understand I can encrypt the information, and the user's passwords,
so that if the database is compromised it still shouldn't be possible
for an outsider to view the confidential information.



However, what about people who have legitimate access to the database,
such as the DBA, Web developer, etc., but who should not be able to
view the confidential information?  For example, even though the
user's password was encrypted, what would stop the DBA from replacing
the user's password with his own (encrypted) password, then logging in
and viewing the user's info, then copying back the original encrypted
password?  Or, adding a new user for himself with whatever
permissions he chooses?