This plugin allows your readers to add tags to your posts (like Flickr or del.icio.us).
The truthful answer is I'm not completely sure. I guess there's not much benefit to spammers since tagging doesn't provide a link. My blog (only a small fry) has had it up about a year and hasn't had a problem. However if you do have a problem, you can easily delete all tags from a given IP address. If you have a serious problem, please let me know and I'll work in some sort of blacklist or (annoying) captcha or something. Also you can delete all tags from a given IP and limit users to X tags per Y seconds.