No, there is no way you can prevent a user from saving what is already been downloaded to the browser.
You can make it "harder" for the user by adding a div on top over your canvas element and set its background to the image (or if possible just apply a css background to the canvas element itself).
You can disable the context menu as mentioned in another answer but the user can override this or use a browser that doesn't care (not sure if Opera has fixed this at the moment..).
You can also generate a bunch of layers and split your map file into many tiles, download those to the browser and draw a tile on separate layers. This is more complicated and makes it a bit more boring for the user to have to download all tiles and stitch them together.
But conclusively, no you can't prevent this - the user only need to go to console and network to find the downloaded image and save it out to the disk.
