Be very careful about what you do with the decompiled source code as some of them may be copywrited and trying to use these source codes entirely on its own or pass them as your own can really be HUGE trouble for you.
Nothing will stop decompilation but the usefulness to human readers of the generated code can be made pretty poor :)
And indeed NEVER pass off decompiled code as your own, and NEVER decompile code you didn't get explicit permission for to decompile it.
Such code is copyrighted to someone else and may contain patented information you have no right to.
And if you do get permission to decompile something often you'll get the source as well :)