Can we undo the favor God has given us?

Yes.  It is possible to undo the favor that God has given us with an individual or a group of individuals.  You say, how can this be?  It happens when we behave our way out of the favor that God has given us.  In other words, we don't behave as we should with the individual that God has given us favor with.  For example, when the person becomes high maintenance or is too demanding with the individual that God has given them favor with, one can loose this favor.  I think the story of Esther really points this out nicely.  Let's take a look at Esther 2:15 (AMP) where it says, “15 Now when the turn for Esther the daughter of Abihail, the uncle of Mordecai who had taken her as his own daughter, had come to go in to the king, she required nothing but what Hegai the king’s attendant, the keeper of the women, suggested. And Esther won favor in the sight of all who saw her.”  Notice, that Esther who was in the Kings Palace and had experienced all of the privilege that came from being a part of the Kings palace, required nothing additional over and above what was provided.  In other words, she wasn't a high maintenance person or a person who was very demanding.  It was her attitude and disposition that won her favor in the sight of everyone.  Even in the sight of the other girls who were competing with her in a sense.