"kevin is her weakness.
karodium is said to drain the "magic" out of people."

"Remember Gwen said she could not track mechanical devices. I think an electro- mechanical device should work.
"

"Another way to keep Gwen passive would be to connect her to a monitoring device that every time she tries to exert her powers some kind of deadly spiked or bladed object inches closer to Kevin. Of course that would depend on keeping Kevin from using his powers to armor up against it. Maybe a metal press would be more effective, since the sheer pressure of the machine would be able to overcome his armor.

Keeping Gwen drugged so that she couldn't concentrate or focus on using her powers would be another good way to keep her quiet. If she can't think or even stay awake then she can't free herself.

Connecting her to a power-draining gizmo of some sort could siphon off her mana as fast as she tries to use it, leaving her unable to get any use out of her power. Of course if she managed to feed enough mana into it fast enough to overload it that plan could fail.

There's probably tons of other ways to keep Gwen prisoner. All it takes is a little imagination to think of a few."

"One limit to Gwen's powers is her imagination. If she can't imagine a way to utilize her powers to free herself then she won't be able to do it. Of course that's not a big limitation, but it's something to keep in mind. Gwen hasn't had any official training in the ways to utilize her powers. She has to make it up as she goes. A trap a fully-trained Anodite would find laughable can hold her if she has never thought of a way to use her powers that would work in that situation.

There seems to be a limit to the "strength" Gwen can exert through her powers. Sure, she can toss around a couple of DNAliens that are holding her, but clamp her wrists to a strong metal or stone wall and her powers won't do much to hurt it. Also, remember in "Inside Man" when Gwen used her powers to lift that teleport key out of the back of the ruined truck and had to strain with all her might just to hold it up? That gives you a sort of benchmark as to just how much force Gwen can exert with her powers.

The EM field is a good idea. Use a mechanically generated energy field to cut off her access to mana from the living world around her and her powers should dry up. She'd have to hoard whatever mana she had stored up until she could find a way to make it count because once she used it up she couldn't get more. Keeping her in a room filled with machines and metal and plastic and concrete with no other living things around would have a somewhat less dramatic impact on her powers, but it should weaken them noticeably."

"so her main weaknesses are 1.technology 2. no life 3. concentration 4. something that drains out her powers! "