The SWF previewer would likely still result in IDM providing a download option, but the downloaded file will be a reduced quality rendition of the original document.
Technically, as long as you expose any previewer capability, IDM is likely to provide something to download. Even if that is only a rendition, the URL provided can be manipulated by users to still download the original document. Everything you are doing to restrict the ability of download is only UI focussed. Technically, if a user has READ access to a document inside Alfresco and is somehow able to piece together the URL to it, they can download it (the URLs aren't that complex either).
Regards
Axel