I see what you mean. There is a trick you can use, although it's one of those bottom-of-the barrel ones that'll have a limited effect, if any. It certainly won't work on the deep engines that don't read ALT tags which is the majority.
Make a one-pixel (1*1), transparent graphic and load the ALT tag with keywords, as you've described. Place it anywhere you like on your pages because it's invisible, it's quick to download (only a dozen or so bytes!) and even if the user has their graphics switched off, they won't see any of the spam because the space for the image is so small. Don't forget to use WIDTH=1 HEIGHT=1