Encrypted column - limit key access
All versions prior to 2016 (currently in CTP/beta) doesn't allow you to easily lock DBAs from the data. Column encryption that you employed stores the key in the DB and secures it using the key hierarchy within the database which roots itself in the service master key. That means you can't lock it away from DBAs. The only way out is to use extensible key management (ekm) to store your encryption keys outside of the database. That allows you to cut DBAs out of the loop but it implies your users are either technically savvy or you have done a great job integrating the user's application with the ekm provider. You can read up more about ekm and implementation options in the links below


https://msdn.microsoft.com/en-us/library/bb895340(v=sql.110).aspx
https://msdn.microsoft.com/en-us/library/dn198405(v=sql.110).aspx
https://www.thales-esecurity.com/knowledge-base/solution-briefs/thales-microsoft-sql-server-ekm
http://www.safenet-inc.com/data-protection/database-encryption/sql-encryption/


If you can upgrade to SQL Server 2016 (still in CTP/pre-release but by the time you test, upgrade, test, modify app, test it'll probably be released), the new Always Encrypted feature makes this a lot easier. Encryption takes place on the client (app) side and the master key is stored client side. Server will only ever receive cipher text so DBAs are essentially blocked even if they run server side or network level trace. However, that's only in 2016 and there's no indication it will be backported to earlier versions (rarely ever happens).