Currently after creating a wave and inviting somebody one cannot kick users out of a wave.
So the idea was to build a gadget/robot which handle a body list of wave users. The wave will get encrypted with every change the key will change.
After kicking an user out of the wave, the key of the user will not be updated, so he will not longer be able to participate at the wave.
Practically it may be possible to insert new unencrypted content to the wave, but the encryption mechanism will automatically hide unauthorized messages.