In the blog post "How I shut down comment spam on this site", Jeff Croft explained how he used hidden fields and client side javascript timers to determine the possibility of spam submissions from bots.

I decided to take a look at how this implementation could be achieved within ASP.Net MVC. To read about this implementation and to grab a demo project in either C# or VB.Net then head over to the article hosted on The Code Project;

http://www.codeproject.com/Articles/325032/MVCBotWatch