keeping data safe offline

I am developing a software that is going to be distributed to end-users on usb sticks. The application and the content will be stored on that device and the content will be stored in a one-file sqlCE database, it will be crypted by default and will be encrypted by the application on-the-fly.
My client has made it clear, that he wants to keep end-users from copying the content and using it on any other device but that very stick. Now, due to the offline requirement this is impossible to achive because i have to store the encryption key somewhere in the code and users are able to access the data while in unencrypted state. 
Can anybody recommend me any mechanism that i could apply, to make it more difficult for users to copy the content? 

ERKI


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Securing Apache Web Server with thawte Digital Certificate
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