yes ive seen a plugin that lets you control the sidebar that might work to limit where they are shown
Using a plug-in is one solution. I actually hard coded (very easy to do) an alternative sidebar just for posts where I do not have the Blogroll and MyBlogLog widget that I have on my home page sidebar. So yes, they are. As everything else in your sidebar (archives, categories, etc). But you can easily change that.
Depends on the theme, some themes like the default theme don't show the sidebar on single posts. But like gbacchin says it's very easy to modify yourself.