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.

If you know which sites that you want on your Top Sites, you could fill the grid with pinned sites. This way, more sites would not be added since they would not be able to replace pinned sites.