The problematic abilities are Evil Eye and Curse of the Evil Eye. Luckily we have the following.
Evil Eye: The fomorian magically forces a creature it can see
Curse of the Evil Eye (Recharges after a Short or Long Rest). With a stare, the fomorian uses Evil Eye,
So a hunting party goes out and overpowers a fomorian, the first thing they will do after it is incapacitated is put a iron hood over it's head that blinds the formorian.
The only way to make it safe for public spectacles is either blind it and train it to blind fight. Or for the formorian fight make sure crowd is more than 60 feet from the formorian. Perhaps by dragging out or erecting a large cage in the middle of the arena. The formorian likely will be wheeled in hooded and immoblie. The mechanism would have a long chain or a enclosed room with a lever so somebody in the arena can release the Formorian from the restraints and the hood for the fight.
Either way it going to have to be setup so that that arena operators earn their investment because all this will be very expensive to implement. It would be far cheaper to capture a giant, disfigure it and call it a fomorian rather trying to deal with a real one.