Is it possible to password protect your SQL Server database?

Question

During my visit to a prospective client who wants me to upgrade a windows application (Rice Mill Operation) done by a certain developer whom they cannot reach anymore. When I log in to their SQL Server using the administrator account, I was amazed that he (previous developer) was able to password protect his database. I've done several googling about passwording a SQL Server database and all of them says it can't be done. 

 

My question is, how did he do it? How to password protect a database from a SQL Server?