In 325 AD Constantine passed an edict against gladiators (Codex Theodosianus book 15, title 12, leg. 1). The version in Cod. Just. XI. 44 runs:
Bloody spectacles in a time of civil peace and domestic quiet do not meet with our favor, wherefore we absolutely prohibit the existence of gladiators.
But clearly nothing happened.
In 403, Prudentius in Contra Symmachum book 2 appealed to the emperor Honorius to abolish the gladiatorial games:
Then on to the gathering in the amphitheatre passes [the vestal virgin] this figure of life-giving purity and bloodless piety, to see bloody battles and deaths of human beings and look on with holy eyes at wounds men suffer for the price of their keep. There she sits conspicuous with the awe-inspiring trappings of her head-bands and enjoys what the trainers have produced.
What a soft, gentle heart! She rises at the blows, and every time a victor stabs his victim’s throat she calls him her pet; the modest virgin with a turn of her thumbbids him pierce the breast of his fallen foe so that no remnant of life shall stay lurking deep in his vitals while under a deeper thrust of the sword the fighter lies in the agony of death.
Does their great service lie in this, that they are said to keep constant watch on behalf of the greatness of Latium’s Palatine city, that they undertake to preserve the life of her people and the wellbeing of her nobles, let their locks spread nicely over their necks or nicely wreathe their brows with dainty ribbons and lay strings on their hair, and below the ground in presence of ghosts cut the throats of cattle over the flames in propitiatory sacrifice, and mutter indistinct prayers?
Or is it that they sit in the better seats on the balcony and watch how often the shaft batters the bronze-helmed face with blows of its three-pronged head, from what gaping gashes the wounded gladiator bespatters his side of the arena when he flees, and with how much blood he marks his traces?
That golden Rome may no more know this kind of sin is my prayer to you, most august Head of the Ausonian realm, and that you would command this grim rite to be abolished like the rest. See, has not your father’s merit left this space unoccupied, and God and your sire’s kindly affection kept it for you to fill up? So that he should not take for himself alone the rewards of his great goodness, he has said “I keep back a portion for you, my son,” and left the honour for you undiminished and unimpaired.
Grasp the glory that has been reserved for your times, our leader, and as your father’s successor possess the credit he has left over. He forbade that the city should be wetted with the blood of bulls; do you command that the dead bodies of wretched men be not offered in sacrifice. Let no man fall at Rome that his suffering may give pleasure, nor Virgins delight their eyes with slaughter upon slaughter. Let the ill-famed arena be content now with wild beasts only, and no more make a sport of murder with bloodstained weapons. Let Rome dedicate herself to God; let her be worthy of her great emperor, being both mighty in valour and innocent of sin; let her follow in goodness the leader she follows in war.
Then the following incident, described in Theodoret, Church History, book 5, chapter 26, took place:
Honorius, who inherited the empire of Europe, put a stop to the gladiatorial combats which had long been held at Rome. The occasion of his doing so arose from the following circumstance.
A certain man of the name of Telemachus had embraced the ascetic life. He had set out from the East and for this reason had repaired to Rome. There, when the abominable spectacle was being exhibited, he went himself into the stadium, and, stepping down into the arena, endeavoured to stop the men who were wielding their weapons against one another. The spectators of the slaughter were indignant, and inspired by the mad fury of the demon who delights in those bloody deeds, stoned the peacemaker to death.
When the admirable emperor was informed of this he numbered Telemachus in the array of victorious martyrs, and put an end to that impious spectacle.
This seems to have taken place in 404 AD, although I don’t know on what that date is based.
The ban of Honorius, naturally, only applied in his presence. I don’t know if we have evidence for later gladiatorial combats. But Salvian tells us in De gubernatione Dei, book 6, that men were still being eaten by beasts in the arena in his day.