Google Whuffie Problem

I just understood a strange thing I was seeing in my blog comments from time to time: someone would post an inane or short unrelated comment on an older post. It hit me what they're doing.If you rank high on Google -- i.e., have good Whuffie from other sites -- then posting a comment with the URL of your horrible, bad Whuffie site means you magically inherit my Whuffie in part. Foo. I'm not sure how to prevent that, although I'm sure Google's algorithms account for at least part of this effect.