Normally if you include originals they will be allowed to download. If you select to link to scaled images, you can still select some originals to be included with right-click -> Include original. So you can only allow originals positivley but not negatively.
Use the "Settings / Photoblogger / Lightbox / Captions/Buttons / Download" to allow the download button to appear.
Yesterday I fixed a bug related to the download button. I also removed the "Disable download button", as this was a duplicate option of the above.