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:

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.