Reservations ('I think this may be a mistake but I can live with it.")
Standing aside ("I personally can't do this, but I won't stop others from doing it. ")
Blocking ("I cannot support this or allow the group to support this. It is immoral." If a final decision violates someone's fundamental moral values they are obligated to block consensus.)
Withdrawing from the group. Obviously, if many people express non-support or reservations or stand aside or leave the group, it may not be a viable decision even if no one directly blocks it. This is what is known as a "lukewarm" consensus and it is just as desirable as a lukewarm beer or a lukewarm bath.