In the year 390, that is.
“There is a remarkable event that took place between the Emperor Theodosius and St. Ambrose, Bishop of Milan. What makes it remarkable is that it shows an 
ancient Bishop interacting with an ancient politician over the dignity of human life. In this case the politician was the Emperor and he had the power of life and death over Ambrose the Bishop. St. Ambrose knew he had to correct the Emperor but also knew this might endanger his life or freedom. Nevertheless he did it and wrote a personal letter of rebuke to the Emperor. Let’s look at this remarkable incident, what Bishop Ambrose did and what the outcome was.”
Ambrose barred him from the Eucharist. Excerpt:
“Theodosius, was mortified and went to Milan to seek solace from St. Ambrose. But Ambrose, fearing the Church was just be used as a political prop or fig leaf left the city before Theodosius arrived and in effect refused to meet with the Emperor. This surely endangered Ambrose for it risked inflaming the Emperor’s infamous temper once more.
Ambrose then wrote to the Emperor a private letter (now known as Letter 51). You can read the whole letter here: http://www.newadvent.org/fathers/340951.htm The Letter is a respectful but clear call to public repentance by the Emperor and a refusal to admit him to the Holy Sacrifice of the Mass or to celebrate it in his presence until such public repentance had occurred.”
A Deacon's Wife 