Stop full post from showing (another user, another problem)

I've read a thread with the same title, but my problem is different.

I run a blog aggregator (www.gameblogs.com.br). Videogame blogs owners register in my site so it can syndicate their content.
I've changed every link in the home/category excerpts so it points out to the original post in the original site. This way, there is no sense in having the full post being shown anywhere in MY site.

The problem is that search engines index the entire post in my site, and provide users with the link to the full post in my site. This is the only way a user can view a full post in my site.

I want that even if viewing the single post page, the user will see just the excerpt with the links to the original post. Is this possible?

If I was not clear enough, please let me know.

Thanks a lot!