Earlier this year, we discussed the possibility of page sculpting or siloing where you point your link juice to the more important pages and nofollow the less important pages. A month after the original post came out, we looked into the possible penalties for controlling that link juice.
The issue is being revisited again on WebmasterWorld. Why do people nofollow their unimportant pages? As stated in the previous paragraph and as reiterated by Tedster, it's so that the "target URL cannot play a part in circulating PageRank for the site."
But some forum members, likely accidentally, suggest that adding a meta tag for "noindex, nofollow" might be sufficient. As Tedster explains, those pages will still get PageRank, so it's not the same as applying rel="nofollow" to a link. Lord Magestic seconds this sentiment:
nofollow in META and tag have 2 very different purposes - the first one is to actually prevent bots from following those links (ie crawling), where as the second does not hold such prohibitions and used to reduce value of backlinks for PR-like calculations.
Of course, if someone else links externally to these internal pages, you might be out of luck, but the question, after all, was about controlling internal link juice.
You can also remove the Sitelinks through the Google Webmaster Tools, as another forum member suggests. But the downside to doing it that way is that you can't suggest a link replacement. Once you remove a link, it's deleted. So exercise this with caution.
The interesting forum discussion continues at WebmasterWorld.