NOPE.
SA is GOD. Anyone with the SA password can do anything (as long as you are using SQL Server Authentication).
A user can only access your database if they are given permission to access it. Only someone with DBO, system administrator, security administrator privilege OR SA account can add/change/delete database access. (Be aware BUILTIN/ADMINISTRATOR account gives system administrator priviliges to ANYONE who is a WINDOWS ADMINISTRATOR. It is HIGHLY suggested that you delete that account BUT only after you have given system administrator privilege to the proper users. If no one is sysadmin on the database/server, and you forget the SA account password and you delete BUILTIN/Administrator account - you probably will not be able to administer the database.)
So, let's say I have access to the SA account. I have access to your database and I can give anyone else access. If I am a DBO on your database, again I can give anyone else access. If I am a Windows Admin and BUILTIN/Administrator account exist - yep, I can give anyone access to your database.
No matter which way you do authentication, anyone with the Server roles of system administrator or security administrator can add/remove/change logins.
You as DBO need to check logins daily to make sure none are being added/removed/changed without your permission.
-SQLBill