Google is slowly but steadily adding all of the basic features into Wave. Recently they added email notifications, and not long before that they added access permissions. Now they’ve knocked out another one of those “must have” features; being able to remove someone from a wave.
You can read more about it on the Google Wave Help site, but it’s pretty simple:
To remove a participant, click his or her picture at the top of the wave, and then click Remove. If the participant you remove has previously opened the wave, a read-only copy of the wave, with a large red “X” at the top, will remain in his or her account, but it will only contain content up to the point of removal. He or she will not have access to any content added after you that point, and they’ll no longer be able to contribute to or edit the wave.
If you remove someone from a wave before they open it, the entire wave will simply disappear from their account. It isn’t possible to see whether or not any user has already opened a wave.