You really cant. If someone can decompile your dll or application back into the source files, there isn't much you can do in that regard. (Of course, the person would first need to have access to the compiled application in question to do this.)
For example: http://www.remotesoft.com/salamander/
this works very well and a single license for it costs 1100.00 (You can demo it on their website though.)
I should point out that you could use Dotfuscator which will obfuscate your compiled code but the above link claims to unobfuscate any code that may have been compiled like this.
I will only tell you how to do it, not do it for you.
Unless, of course, you want to hire me to do work for you.
Read this if you want to know how to get a correct reply for your question:
^^Took that from planoie's profile^^
^^Modified text taken from gbianchi profile^^