Yesterday I had the fortune of seeing John Vennari at our chapel where he gave a very good lecture for a couple of hours. Afterwards there was a brief question period where someone asked if he would ever consider the see of peter to be vacant. He said no. Anyhow in his explanation he stated the possibility of all the bishops coming together and forming a council whereby the say to the Pope you have said X, this is the definition which declared X to be heresy, now knowing this do you still believe X to be true. and if he answers in the affirmative and dosn't renounce X then he is declared deposed. *Whereby X representing a heretical position*
Now me speaking, it is also possible to wait for another Pope to condemn him or convene a council to condemn him like Honorius.
Laymen cannot condemn a Pope, Lay cannot even condemn a Bishop, but if they utter things that contradict the magisterium then we must use our intellect to discern right from wrong and disobey that which is wrong.