I've installed Dropbox on my new computer and accepted the suggestion to sync my Desktop, Documents and Downloads folder. However these folders filled my storage and I want to erase them (I don't want this sync). I deleted the My Computer folder from my online account but still my account is full. When looking in my three existing folders they should be around 1 Gbs and not the 2 Gbs Dropbox says I have.
It seems like you enabled the computer back up feature on your Dropbox application.
You can disable this feature, by following the steps here.
If you want to free up space by deleting files, please make sure that this feature is disabled first, because if you delete them from Dropbox, while the feature is enabled, they will be deleted from your desktop as well.