googlebot using internal search

I recently installed the Search Unleashed plugin. It offers a log of searches performed. This is showing Googlebot using the internal search function of my WordPress installation.

I don’t think I want this.

I see that the Meta Robots plugin can prevent indexing of search result pages. But I also see that using a robots.txt file is supposed to be able to do it too (the Meta Robots plugin seems to do it some other way).

a) how would I do that using robots.txt?
b) is that preferable to using Meta Robots plugin?
c) any reason why I should NOT do it at all?

Note that I have submitted a sitemap to Google Webmaster Tools so I’m thinking the internal search results don’t add much.

Thanks