I play the Oath of Vengeance paladin and I've came across a problem.
There is the Channel Divinity: Abjure Enemy description, which states:
As an action, you present your holy symbol and speak a prayer of denunciation, using your Channel Divinity. Choose one creature within 60 feet of you that you can see. That creature must make a Wisdom saving throw, unless it is immune to being frightened. Fiends and undead have disadvantage on this saving throw.
On a failed save, the creature is frightened for 1 minute or until it takes any damage. While frightened, the creature’s speed is 0, and it can’t benefit from any bonus to its speed.
On a successful save, the creature’s speed is halved for 1 minute or until the creature takes any damage.
It sounds pretty good, but does anything forbid an abjured enemy from simply harming itself on its turn?
The enemy doesn't know the rules of Dungeons & Dragons.
The only reason you have thought to ask this question is because you read the feature description of Abjure Enemy. But the creature affected by it has not, and so shouldn't even think to try this. Sure, technically if the creature successfully damages itself, it can end the effect of Abjure Enemy, but there is no reason in the narrative that they would know or think to do that.