There is already a button with the behaviour "mark all blips as read in this wave". Within a wave, right under the list of participants of the wave, there are several buttons. One is "Read". This should work for this issue, right? Notice there is a small bug with that button (http://redmine.ourproject.org/issues/516 ).
When you join to a old wave, all menssages at him become notifications.We need to change this behaivor or add a obtion to bulk mark this bits as read.