Recently one of my clients - who are still believing in "Page Rank Sculpting" whatever this may mean - asked me to stop Google from indexing some of links Joomla generates. Most of them are easy to "patch", using various tools (sh404SEF, Marco's nofollow plugin), the most problematic one (for me, of course) was the E-mail this button on the top of the article.
A brief look to the code has "delivered" the solution, which is simpler, as you may believe:
Open /components/com_content/helpers/icon.php, and look for the functions which are generating those links. You can find easily, that the "function pdf(...)" for example has a line which is missing from the "function email(...)":
$attribs['rel'] = 'nofollow';
around line 62. You have nothing more to do, just simply copy that line, and place it in the needed places, just after this line (Line 85 in Joomla 1.5.23):
$attribs['onclick'] = "window.open(this.href,'win2','".$status."'); return false;";
Now Google is properly warned about your intention to stop crawling the link. If this will have or not the desired effect is yet to be seen, some SEO forums are reporting for years by now that Google simply ignores the "rel='nofollow'" and decides on it's own what will do. But you at least tried.