Do you know how the spam posts were published on your site? Just make sure the vulnerability is fixed so it doesn't happen again. Once the spam posts you found have been deleted from your site, you shouldn't have to do anything more since they will fall out of Google's index. Keep an eye on Google Webmaster Tools though to see if you notice any more spam pages pop up on Google's radar and then manually remove them.
Here is Google's official answer - http://support.google.com/webmasters/bin/answer.py?hl=en&answer=164734
When a page is updated or removed, it will automatically fall out of our search results. You don’t need to do anything to make this happen.
However, if you urgently need to remove content from Google's search results (for example, if you’ve already removed, updated, or blocked a page accidentally displaying confidential information like credit card numbers), you can request expedited removal of those URLs.
Our removal tools are intended for pages that urgently need to be removed—for example, if they contain confidential data that was accidentally exposed. Using the tools for other purposes may cause problems for your site.
Another Google resource if your site was actually hacked or compromised - http://support.google.com/webmasters/bin/answer.py?hl=en&answer=1269119
To take your site "offline" after being hacked. If your site was hacked and you want to get rid of bad URLs that got indexed, use the URL removal tool to remove any new URLs that the hacker created—for example, http://www.example.com/buy-cheap-cialis-skq3w598.html. But we don't recommend removing your entire site, or removing URLs that you'll eventually want indexed. Instead, clean up the hacking and let us recrawl your site.