Block site from being on Safari Top Sites
From Apple's support page: On the Top Sites page, click the Edit button. Then do any of the following:


To specify that a page be kept in Top Sites and not replaced by other pages visited more often, click the pin icon in the page’s upper-left corner.
To reverse this action, click the pin icon again.
To specify that a page never be included in Top Sites, click the X in the page’s upper-left corner.
To display 24, 12, or 6 webpage previews, click Small, Medium, or Large.
To rearrange the pages, drag them.


When you finish making changes, click Done.