I spotted a thread started by one of the webmasters of Dun & Bradstreet at the Google Webmaster Help forum where the D&B webmaster said they have blocked a subdomain from showing up in their sitelinks but it is not going away.
The story is a bit interesting actually.
D&B sold a subdomain of theirs to a competitor about two years ago. That subdomain is something they do not want to advertise, especially within their sitelinks. So they used the block sitelinks feature.
In the picture below, the arrow points to the sitelink that links to their competitor.
The webmaster wrote:
About 2 years ago, we sold one of our subdomains to another company. That company's links still show up in our sitelinks, so we have set them to 'Block' in GWT. About every 90 days I re-block them in GWT. The 4 sitelinks I currently have blocked all belong to this company.
The last time I updated the blocks was 7/20.
Today I noticed a new sitelink in Google's SERP, and it points to the competitor's site - http://bit.ly/ro3Pqi. In GWT there is no option to block this link.
Google's Gary Illyes responded saying they found the issue and hope to resolve it soon. Gary said:
We've identified the issue and we're working on a fix. Unfortunately I can't say a timeframe for the fix will show up in the search results, but it should happen fairly soon.
Pretty interesting case, don't you think? I am not sure if the whole sitelink blocking feature is broken or just this one.
Forum discussion at Google Webmaster Help.