This really depends on your theme and how it's set up. Some show categories, some show pages, some show both.

There's a plugin called 'Exclude Pages', which will get rid of pages. For categories, there are similar plugins, but I've always had trouble with them as most I found were for older versions of WP. I've always used this guide to exclude categories (if not an option in your theme of course...): http://blogmum.com/2009/04/how-to-ex...ordpress-blog/

If you could list your specific theme, it might be easier to provide a more exact answer?

Another option is in the newer version of wordpress they use a new feature called menus. Generally all newer themes that are WP 3.0 compliant can utilize this. You make your custom menu with only the pages you want to show navigation for and then set that menu as your main navigation. Therefore only pages you add to that custom menu will show.

Again this is all theme dependent.

The "exclude pages" plugin is great for that...

WordPress › Exclude Pages « WordPress Plugins

Best way to remove navigation is to tweak the code in header.php
I am not a coder and I figured out how to do it. Probably can get someone on Fiverr to do it for you if you are scared to try.

I have always read not good to have too many plugins.

If the navigation is in the header then you can try editing the header.php file in your theme directory. If its a sidebar you might be able to tweak menus or the page.php file in the theme. If there is no page.php file them make a custom one if you know how.

All recommendations vary based on the theme you use though.

Cool. Thank you!

Please go to the wordpress user manual. They can help accurately.