This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
If you really want to make it impossible for anyone to reverse engineer your code, then you could make it a web application that runs on a server. People using the web app through their browsers will not be able to get at the code that is running on the server.
If it has to be code running on the client's computer, then there is no way to 100% guarantee that nobody can get at the code. There are some products, called obfuscators, that make the code harder to read, but someone who really wants to could in principle still reverse engineer it.