You can customize the toolbars in calibre, hiding the unneeded functionality, see Preferences ==> Toolbars.
But you cannot strip it out of the program itself.
You can install the portable version of calibre, load the books into the library, and give them that. Clicking an icon of a stack of books would definitely be easier for non-tech-savvy people, although you might be surprised how many get the hang of "these 'file' things" once they have to.
As far as protecting the product you sold from your tech-savvy customers, as a tech-savvy customer I am diametrically opposed to that! Sounds like you want DRM, which will cost you quite a chunk of money for an Adobe vendors license. And all the common DRM encryption schemes have been cracked, and come with a point-and-click method. (See:
https://apprenticealf.wordpress.com)
Someone who desperately wants to "share" will do so no matter what you try. Offering the customer a pleasant experience (on their terms) encourages goodwill, and further purchases vs. pirating, generally speaking.