Recently my Joomla site that uses K2 started getting a lot of "Soft404" errors. The URLs look like

where xxx is a random number. The referrer in these cases is my own website so I assume someone is spamming me via an attempted search and trying to find a way in. What I want to do is return a 404 in these cases instead of a 200. It seems the page is dynamically generated and returns a 200 but with an error inside it like this

I don't really need to fix this page as much as I want the header to include the 404. Can anyone point me in the right direction?

This can be achieved with Joomla!'s user user object in your overrides.
If the id does not match a user, then you can raise an error page.
docs.joomla.org/Accessing_the_current_user_object

The page is already raising an error but it's returning 200 OK in the header. This is what I want to fix.

You can redirect the user to a 404 error page with that method.