In this other question, I've asked how to remove a domain from being added to the Safari Top Sites.
Unfortunately it seems that the answer may not be that easy as I though. I've just discovered that the list with the banned domains is on the path
~/Library/Safari/TopSites.plist, and I tried to add a domain with a wildcard to the BannedURLStrings, i.e.
http://apple.stackexchange.com/*, but it doesn't work. Safari removes it as it seems to be invalid. Someone knows if it is possible to add a wildcard, or if they have another syntax, i.e