Check its equipment to see if anything makes the user Berserk. Also check the traits - I don't think any of them provide Berserk, but I could be wrong. If there is a piece of equipment that has the Berserk effect, unequip it and you should be able to control your champion again. If the problem does not go away, then make sure you unequipped every piece of equipment with that property, check enchantments to make certain that those don't have that property, and check traits.

If you need to dispel an enchantment to get rid of this:

- Friendly enchantments can be dispelled from the Kingdom Report (double-click on one of the resource icons and it will come up; double-clicking the mana resource icon will bring you to the correct tab), and I think also from the unit details screen.

- Hostile enchantments require a spell which I think is in one of the second or third tier Magic techs.

If you need to remove a trait, then you're out of luck without making use of mods or cheating, and I couldn't tell you how to go about creating a trait-removal mod or using a trait-removal cheat.