Google Wave has added the ability to remove participants from any wave. It's a somewhat complex feature (you wouldn't want someone to be able to, say, delete your email or shared documents), but Google came up with an elegant solution.
Depending on the situation, removing a participant from a wave will work in different ways. If the person hasn't already read your message, the message simply disappears:
Imagine this: after a rough day at work, you spend the night out with friends and return home to write a less-than-flattering message to, say, your boss. At 6am you wake up and realise what you've done to your implacable horror! Alas, with email and other forms of communication, you'd be out of luck, and perhaps spend all morning inventing clever excuses about how your computer was taken over by crafty hackers. With a wave however, you simply remove your boss as a participant, and when she wakes up at 7am to check her messages, the wave is gone from her inbox. As long as she didn't open the wave before you removed her, she will never see it. Cool!
Imagine, on the other hand, you had been collaborating on a document with another Wave user, and that person decided out of the blue to remove you from the Wave. You wouldn't want to lose all access to the document, so Wave gives you a copy of the document up to the point that you were removed from the wave.
Consider a different scenario: someone adds you to a wave promising they'll fetch the water if you'll whitewash their fence. You open the wave, agree to the proposition, and proceed with the whitewashing. Yards of fence later, that someone removes you from the wave. You certainly would want to have a record of the deal ('It totally happened')! Since you had opened the wave before you were removed from it, you retain a read-only copy of the wave up to the time you were removed. You can still play back the wave, but you won't see future changes or be able to make further contributions.
The lack of this feature has been a glaring feature gap in Wave since its (closed beta) release to the real world, but it's good to see they came up with a smart solution to the problem.
Removing participants [Google Wave Blog]