SSIS Package Security
This is not native functionality. You can encrypt: Nothing, Sensitive items, Everything. There is no capability to assign one key for development and one for execution. 

Instead, the way to restrict execution is through proper security, which is never the last item. If the SSIS package is loading a database, you ensure that only the correct user(s) can do connect/write to the database. If you store the packages in the database, deny them the ability to run the packages. If it's on the file system, set the proper ACL so they can not get to them.