Nope. There's really not. Askimet can make it so they don't bother you, but it won't stop it. Really, it's not illegal. Full data posts POSSBLY, but still, very very low. I know for myself at least, there's no way to go through my splogs to contact me, even for a copyright complaint. But as for ones that give a backlink, it's the same as a person writing an essay and citing their source. Nothing you can do.