Fundamentally there is no way to prevent the client seeing the code that you serve, since this is what the client requires to build and render the page to the user. Even if somehow you disable the developer tools, it's still possible to access the code in other ways.
You can employ certain methods to make it more difficult to understand the code sent, but a determined coder can re-engineer from minified code. If you apply good build processes however, then you will strip out all your comments and explanation of how it works.
For UI5, there is a built in mechanism for switching between debug sources (with *-dbg.* in the filename) and the compressed files. I suggest for any public facing UI5 app that only the minified sources be included in the BSP repository.
And as others have said, the next step is hardening the web server the app is served from (be it Gateway/ABAP, Netweaver Java, open-source, etc.). If this is what you are after, then suggest raising a separate post with specifics.