Redirect to main view URL when no contextual filters are set
The problem lies in Drupal's menu system, which for the most part asserts itself as a safety mechanism, so if you or someone else mis-spells a URL it will give you the next best option.

Admin Menu Example

admin/structure/blocks = Manage Blocks Page

admin/structure/blockss = Structure Page

Drupal does this so it won't "disrupt" the user-experience, but it can also be a confusing if you don't know what just happened. 

It's generally there to prevent human error and other problems that arise when managing a live website.

The Options


Keep the default set, so users are accommodated and instead focus on structuring the robots.txt (and perhaps .htaccess) file correctly, so you can let Search Engines know what's happening.
Look into the Rules and Context contributed modules to see if you can create a solution using those interfaces. You may or may not be able to find a solution there, but it might be a good start.
Create a module, which will handle non-existent URL's and correctly reroute them to 404 page.