How can I "hide" the data in AWS from users?

I want to build an application using Amazon Web Services (AWS). The way the application should work is this; I make a program that lets the user import a large file in an external format and send it to AWS (S3?) in my own format. Next many users can access the data from web and desktop applications. I want to charge per user accessing the data. The problem is that the data on AWS must be in an unintelligible format or the users may copy the data over to another AWS account where I can not charge them. In other words the user need to do some "decrypting" of the data before they can be used. On the web this must be done in JavaScript which is plaintext and would allow the users to figure out my unintelligible format. How can I fix this problem? Is there for instance a built in encryption/decryption mechanism? Alternatively is there some easy way in AWS to make a server that decrypts the data using precompiled code that I upload to AWS?