First, you need to assess the URL's that google complains about.
If the URL's contain a bunch of gibberish characters and doesn't seem to fit in with your site, then I'd leave them as 400 status and remove the affected URLs from any sitemaps you have submitted to google. Examples of qualifying URLs should include:
If the URLs contain normal characters and they blend in with your site but the pages at those URLs no longer exist, then the 410 status should be returned so you won't waste google's time trying to index useless pages.
If the URLs are meant to point to existing pages at some point in the future but not at this time, then the status returned should be 404.
Examples of URLs with normal characters are:
I recommend using noindex on thin-content pages returning status 200 (pages with little content) you don't want search engines to index at all.