Microblogging services such as Identi.ca (the one I use) and Twitter (the one I avoid because it is uselessly centralized) have an interesting feature: when you have an account on them and choose to subscribe to someone's feed, that person knows that you are following them.

However, I fear that people may become accustomed to this feature, and may come to believe that they will always know who is following them. Of course, this isn't true: for the many users who publish their microblogging entries to everyone, people can just follow the RSS feed without even having an account on the microblogging platform you're using.

The bottom line is that it doesn't make much sense to ask somebody to stop following your feed. The content of the feed is most likely public, and they can just silently follow you. If the problem is that you do not want to advertise them in your followers list, it's your job to manage the list properly, and remove the people you don't like without having to ask them to leave.