Using only current technology we have already created a way to hold prisoner a group of individuals who can manipulate the elements (fire/heat, earth, water/liquid, metal, lightning/plasma) with a certain level of humanity.
Level of humanity: Prisoners are required to be provided with enough food, water, exercises and human interaction to keep them in relative health and sanity. Nothing can be done to them that would cause permanent physical or physical logical injury.
Notes: Elemental manipulators have to move their body to manipulate the elements. Most do this by moving their hands but with practice they can manipulate elements by only twitching their neck or bodies.
Manipulate the elements by itself does not require a lot of concentration, but to do so with any form of skill or accurate dose. the more complex the action you want the element to do the more concentration you will need.
None of the guards in your prison have elemental powers.