Crain's Chicago has a piece on bloggers quitting (January 17, 2011) with all of the stories of people deciding to give up on blogging because it didn't generate as much business as phone calls, and people giving up blogging because they got a new job or had a kid. Good reasons all; the one I see the most around Ann Arbor is giving up blogging when you are done with graduate school.
The good example that I see from it is the account of Aleen Bayard, who gave up on blogging about New Zealand (New Zealeens) because she moved back to Chicago.
Not every blog has to be a permanent effort. If you write about something, and then no longer need to write about something because the world has changed and you with it, that's fine.“I wrote one post when I got home and realized that I had something unique to say in New Zealand but not as much as a Chicagoan back in Chicago,” she is quoted in Crain's as saying.
If you're done with a project, you're done - no harm in that. There's a useful structure of writing for a finite amount of time (e.g. a year) on a single topic, but if that doesn't do it for you, then simply don't be shy about putting a project to bed. Find the time to write one last post, call out the highlights from the effort, wrap up some conclusions and put together a tombstone that says "I'm done". A well-ended effort is that much more satisfying to read than something that seems to end in mid-thought.