If you would like to remove spam it is a nice to consider Akismet. (Wordpress installations come with default integration.) There is an another widely used one called SpamWipe.
If you would like to check Plagiarism, there are other tools like the one that small seo tools is providing. You can take a look at copyleaks.com too.
There is a good list for API-s to use in your case:
http://blog.mashape.com/list-of-18-apis-to-protect-your-site/
Of course you could use this kind of API-s in your application.