code: public void revokeVoice(Collection<Resourcepart> nicknames) throws XMPPErrorException, NoResponseException, NotConnectedException, InterruptedException {
          changeRole(nicknames, MUCRole.visitor);
      }
docs: Revokes voice from participants in the room. In a moderated room, a moderator may want to
  revoke an occupant's privileges to speak. To have voice means that a room occupant
  is able to send messages to the room occupants.
  
  @param nicknames the nicknames of the participants to revoke voice (e.g. "john").
  @throws XMPPErrorException if an error occurs revoking voice from a participant. In particular, a
  405 error can occur if a moderator or a user with an affiliation of "owner" or "admin"
  was tried to revoke his voice (i.e. Not Allowed error); or a
  400 error can occur if the provided nickname is not present in the room.
  @throws NoResponseException if there was no response from the server.
  @throws NotConnectedException
  @throws InterruptedException