First, www.contoso.com/default.aspx. is NOT part of your team site. It's a placeholder page on your public site that uses the same master page as the teamsite so looks similar. Fortunately comparatively few people use yahoo search and almost nobody uses bing, so it's not much of a liability - but if you want to workaround it, add a redirect to that default.aspx page, so if the link is clicked your real home page is returned.
This is easiest done in Sharepoint Designer.
Go to "All Files" for the public site and open the page (in SPD)
In code view, add a meta refresh or javascript redirect after the <body> tag..
You'll find the codes needed if you Google for them.
Secondly, a real fix for this would have to be done by MSFT. Only they can explain why the default.aspx page exists,, why it doesn't have a "no follow no index" tag and why some search engines are indexing it. And only they can stop that happening.