How can I prevent our IT admins with sysadmin to read data stored inside an encrypted database?
Per the other comments, one way to block them is to NOT have the IT Admins in the sysadmin role.  Sysadmin can override anything pretty much.  

If they really need a high level of access and you dont want to out right encrypt your data, you could try a combination of a custom server role and/or database roles.  As long as you are on at least SQL server 2012 (for the server roles piece), this would allow you to grant them a more controlled level of access to the sql instance.  

The real question is "Do they need SysAdmin?"  If the answer is yes, you'll have to encrypt the data.