code: public void decline(EntityBareJid room, EntityBareJid inviter, String reason) throws NotConnectedException, InterruptedException {
          Message message = new Message(room);
  
          // Create the MUCUser packet that will include the rejection
          MUCUser mucUser = new MUCUser();
          MUCUser.Decline decline = new MUCUser.Decline(reason, inviter);
          mucUser.setDecline(decline);
          // Add the MUCUser packet that includes the rejection
          message.addExtension(mucUser);
  
          connection().sendStanza(message);
      }
docs: Informs the sender of an invitation that the invitee declines the invitation. The rejection will be sent to the
  room which in turn will forward the rejection to the inviter.
  
  @param room the room that sent the original invitation.
  @param inviter the inviter of the declined invitation.
  @param reason the reason why the invitee is declining the invitation.
  @throws NotConnectedException
  @throws InterruptedException