If you were to imprison a witch/warlock...

If you were part of an anti-supernatural extremist organization that imprisons and tortures supernatural beings, like witches, how would you stops a witch/warlock from poofing out? (The magical system is probably based more on mental concentration than anything, and they can't just kill them because I need a warlock character to be there for at least a few months).

How about with threats to the witches family/loved ones? Basically you stay here or I'll kill everyone important to you? By the way you are aware that warlock is NOT sean as an acceptable term by those who practice the arts? A warlock is an oath breaker.

Mental concentration? Give all the guards ear protection and blare distracting noises, or flash strobe lights, or mix drugs that impair focus into the food, or work systematic sleep deprivation into the torture. If they can't concentrate, they can't do magic. Someone might manage to overcome this eventually (and in the process they'd probably become almost impossible to distract under non-prison conditions), but I'm guessing it would take a while.

I agree with screnwriter, maybe water torture?

Keep him drugged ALL the time, to the extent that it's practically inducing artificial coma. That, and emplacing whatever you use for antimagic purposes all around the cell (like surrounding Jedi with ysalamiri)

I agree with screnwriter and want to add that I would change the noises daily, so they cannot get used to it.
Also, maybe change the drugs, too.

A male witch can be called a witch. All wiccans are witches, men and women alike, for example.

rare but naturally ocurring material stifles magic. Rowan wood, spices, et cetera. bonus: the captors are running out of witchbane, which is harvested in October and it's only August, so they use less and less and slowly lose control.