Technically speaking, the only way to make your blogs uninteresting for spammers is to a) downgrade their Google PR or delist them from Search engines; b) depreciate their content; c) close them.
Spammers will keep trying to break-in if you have well-recognized, popular blogs. You can always close comments, but you'd loose the regular readers and posters.
If you are making money out of these blogs, try to migrate them into any CMS which makes for spammers harder to register and post. If you run your own (or just not very popular) CMS, the automated scripts some folks use will not work.