If you're using the developer edition, you can remove content from the filesystem. Also, it depends on what kind of content you're referring to. In the online version, I found that uploaded images that are not being used, will be automatically cleaned.
It would be nice if I could easily do this myself without relying on ESRI to do it. The reason being that I believe in my organisation we get charged for content stored online. So I would like to keep this stored content to a minimum if possible.