There were methods for doing this in earlier versions of Safari but I don't know of one that works with 5.1.
The reason I want to do this is that it takes up a great deal of disk space, and I suspect it slows things down on my older, slower Mac running Lion because of all the extra caching going on. I never use the feature anyway.
Here is the link to an earlier question on Ask Different with regard to Safari 5.0.
I am creating a new question because apparently any methods that would work for Safari 5.0 will not work for 5.1.