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.
How can I disable "Top Sites" in Safari/Webkit completely?
I am creating a new question because apparently any methods that would work for Safari 5.0 will not work for 5.1.
Best Answer
While not a clean solution and not properly preventing Safari from creating the thumbnails in the first place, this apparently works to keep them from taking up disk space:
Incidentally, even going to Safari → Reset Safari… and ticking Remove all webpage preview images will apparently leave this hack intact.
As for Top Sites, there are a few places you can remove it from:
I don't believe the menu item can be removed this easily.