A Google Webmaster Help thread informs us that Google can take about two weeks to remove a blocked Sitelink from showing up in the search results.
Google's JohnMu said:
A change like this can take two weeks or a bit longer, depending on the site and on other factors. I would certainly give it a bit more time :-). Please let us know if you don't see it disappear by next week or so, but from what I can tell it looks fairly normal and nothing appears to be stuck at the moment.
You can block Sitelinks within Google Webmaster Tools by clicking on the "block" button on the right of each Sitelink:
There is also a detailed help document on how to manage Sitelinks within Webmaster Tools.
Forum discussion at Google Webmaster Help.
