Rule 1. Break their links.
Rule 2. Make sure they don't notice the links were broken.

As an example on of how to do it perfectly, take a look at Microdoc-News' old feed. No ' We have moved' post in the discontinued feed. No HTTP permanent redirect. Just a dumb 'human consumption only' HTML redirect.
My newsreader of course didn't interpret this correctly, so I was actually just assuming that feed had gone dead - ontil I checked and found the new feed elsewhere.
Not a very clued move by a guy who's trying to make a living through micro publishing.

The reason I bothered is of copurse the good copy on Microdoc News. For example this instruction on accessing specialized google sub searches is useful, although I'm not sure I think Google's implementation of these smart words is as admirable as the page rank itself.